feat: Add GitLab Package Registry migration support

cmd/registry/migrate_cmd.go

@@ -42,7 +42,7 @@ Example configuration file (config.yaml):
 
   source:
     endpoint: https://source-registry.example.com
-    type: JFROG                    # Supported: JFROG, NEXUS
+    type: JFROG                    # Supported: JFROG, NEXUS, GITLAB
     credentials:
       username: source_user
       password: source_password

module/ar/migrate/adapter/gitlab/README.md

@@ -0,0 +1,185 @@
+# GitLab Package Registry Adapter
+
+This adapter enables migration of artifacts from GitLab Package Registry to Harness Artifact Repository.
+
+## Supported Package Types
+
+- **Maven** - Java/JVM packages
+- **NPM** - Node.js packages  
+- **PyPI** - Python packages
+- **NuGet** - .NET packages
+- **Composer** - PHP packages
+- **Conan** - C/C++ packages
+- **Helm** - Kubernetes charts
+- **Debian** - Debian packages
+- **Generic** - Generic file packages
+- **Go** - Go modules
+- **RubyGems** - Ruby packages
+- **Swift** - Swift packages
+
+## Configuration
+
+### Example Configuration File
+
+```yaml
+version: 1.0.0
+concurrency: 5
+overwrite: false
+
+source:
+  endpoint: https://gitlab.com  # or your self-hosted GitLab instance
+  type: GITLAB
+  credentials:
+    username: gitlab-username  # Can be any username for token auth
+    password: glpat-xxxxxxxxxxxx  # GitLab Personal Access Token
+  insecure: false
+
+destination:
+  endpoint: https://pkg.harness.io
+  type: HAR
+  credentials:
+    username: harness_user
+    password: harness_api_key
+
+mappings:
+  - artifactType: MAVEN
+    sourceRegistry: mygroup/myproject  # GitLab project path
+    destinationRegistry: harness-maven
+
+  - artifactType: NPM
+    sourceRegistry: mygroup/myproject
+    destinationRegistry: harness-npm
+
+  - artifactType: PYTHON
+    sourceRegistry: mygroup/myproject
+    destinationRegistry: harness-pypi
+```
+
+### Authentication
+
+GitLab authentication requires a **Personal Access Token** with appropriate scopes:
+
+1. Go to GitLab → User Settings → Access Tokens
+2. Create a new token with the following scopes:
+   - `read_api` - Read API access
+   - `read_registry` - Read package registry
+   - `read_repository` - Read repository (for some package types)
+
+3. Use the token as the `password` field in the configuration
+
+### Source Registry Format
+
+The `sourceRegistry` field should be the **project path** in GitLab:
+
+- For project: `mygroup/myproject`
+- For nested group: `mygroup/subgroup/myproject`
+- For user project: `username/myproject`
+
+## Usage
+
+```bash
+# Run migration
+hc registry migrate -c gitlab-config.yaml
+
+# Dry run (preview what would be migrated)
+hc registry migrate -c gitlab-config.yaml --dry-run
+
+# With custom concurrency
+hc registry migrate -c gitlab-config.yaml --concurrency 10
+```
+
+## GitLab API Endpoints Used
+
+The adapter uses the following GitLab API v4 endpoints:
+
+- `GET /api/v4/projects/:id` - Get project information
+- `GET /api/v4/projects/:id/packages` - List packages
+- `GET /api/v4/projects/:id/packages/:package_id/package_files` - List package files
+- `GET /api/v4/projects/:id/packages/:package_id/package_files/:file_id` - Download files
+
+## Package Type Specifics
+
+### Maven
+- Packages use `groupId:artifactId` naming
+- Supports maven-metadata.xml, pom files, and checksums
+- Download path: `/api/v4/projects/:id/packages/maven/*path`
+
+### NPM
+- Supports scoped packages (@scope/package)
+- Package names are normalized to lowercase
+- Download path: `/api/v4/projects/:id/packages/npm/*path`
+
+### PyPI
+- Package names normalize underscores and dashes
+- Supports wheel and source distributions
+- Download path: `/api/v4/projects/:id/packages/pypi/files/*path`
+
+### NuGet
+- Package IDs are case-insensitive
+- Supports .nupkg files
+- Download path: `/api/v4/projects/:id/packages/nuget/download/*path`
+
+### Generic
+- General-purpose package storage
+- Any file type supported
+- Download path: `/api/v4/projects/:id/packages/generic/*path`
+
+## Limitations
+
+1. **Project-level only** - Currently supports project-level package registries. Group and instance-level registries are not yet supported.
+
+2. **Pagination** - Large repositories with thousands of packages may take time to enumerate. The adapter handles pagination automatically.
+
+3. **Rate Limiting** - GitLab.com has rate limits. For large migrations, consider:
+   - Running during off-peak hours
+   - Using a self-hosted GitLab instance
+   - Adjusting concurrency settings
+
+4. **Docker/Container Registry** - Container images in GitLab Container Registry use the OCI standard and should work, but may require additional testing.
+
+## Troubleshooting
+
+### Authentication Errors
+
+```
+GitLab API error (status 401): Unauthorized
+```
+
+**Solution:** Verify your Personal Access Token has the correct scopes (`read_api`, `read_registry`).
+
+### Project Not Found
+
+```
+GitLab API error (status 404): Project not found
+```
+
+**Solution:** Verify the project path is correct. Use the full path including groups (e.g., `mygroup/myproject`).
+
+### Rate Limiting
+
+```
+GitLab API error (status 429): Too Many Requests
+```
+
+**Solution:** Reduce concurrency or wait before retrying. Self-hosted GitLab instances have higher rate limits.
+
+## Development
+
+### Running Tests
+
+```bash
+cd module/ar/migrate/adapter/gitlab
+go test -v
+```
+
+### Adding Support for New Package Types
+
+1. Add the package type mapping in `mapArtifactTypeToGitLab()` and `mapGitLabPackageType()`
+2. Create a new handler in `package_handlers.go`
+3. Update the documentation
+
+## References
+
+- [GitLab Package Registry API Documentation](https://docs.gitlab.com/ee/api/packages.html)
+- [GitLab Package Registry User Guide](https://docs.gitlab.com/ee/user/packages/package_registry/)
+- [GitLab Personal Access Tokens](https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html)