Add authentication middleware for SSE/HTTP with multi-user support

[Pnkcaht]

What I Did

• Implemented basic Bearer token authentication for the MCP Gateway.
• Added multi-user token support.
• Logged initialization parameters for debugging (logInitializeParams).
• Updated SSE/HTTP/Streaming servers to validate tokens before serving requests.
• Ensured DCO compliance in commit.

Related Issue

Closes: #370

Previously, Docker MCP worked fine locally but was insecure in shared or cloud environments.
This commit introduces native token-based authentication to allow secure, centralized MCP deployments.

Before / After

Before:

• MCP server assumed a "trust everything" model.
• Anyone who could reach the port could execute commands.
• Custom middleman gateway required for token validation.

After:

• SSE/HTTP/Streaming endpoints now validate Bearer tokens.
• Single-user and multi-user tokens supported.
• Initialization logs now structured and include client identity info.
• Secure access possible without VPN or external proxies.

Diagram (Mermaid)

flowchart TD
    Client -->|HTTP/SSE request w/ Bearer| MCPServer
    MCPServer -->|Check token| AuthMiddleware
    AuthMiddleware -->|Valid token| Handler
    AuthMiddleware -->|Invalid token| Unauthorized
    Handler -->|Respond| Client

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Notes / Future Work

• Middleware is applied for authentication, but SSE/HTTP/Streaming servers may need additional fine-tuning to ensure full coverage.
• Exposing a CLI parameter --token to allow passing a secret key at startup is still pending.
• This commit fully closes the related issue for secure multi-user access.
• Future improvements:
  • Apply authentication middleware consistently across all transport types.
  • Expose --token and support configuration via environment variables for easier deployment.