interface for change provider and update request to include uri

[rashmi-prithyani]

Summary

This PR introduces the ChangeProvider extension interface and refactors the Change model to use URIs instead of simple IDs.

Changes

1. New ChangeProvider Interface (extension/change_provider/)
   Added ChangeProvider interface to fetch changed files from external code review providers (GitHub, GitLab, Phabricator)
   Interface method: Get(ctx context.Context, uris []string) ([]FileChange, error)
   Returns list of files with their change status (added, modified, deleted)
   Provider implementations will be added in subsequent PRs (e.g., extension/change_provider/github/)
2. Refactored Change Model: IDs → URIs
   Changed Change.ids to Change.uris throughout the codebase
   URI format is provider-specific and self-contained:
   GitHub: owner/repo/pr_number@commit_hash (e.g., uber/submitqueue/123@abc123defg)
   Phabricator: revision_id@commit_hash (e.g., D12345@abc123def)
   Updated proto, entity, storage, controllers, and tests
3. Database schema change:

• change_ids column renamed to change_uri
• removed change_source field.

Test Plan

unit tests and ci

Issues