StreamableHTTP - listening for messages from the server trough GET established SSE

[ihrpr]

The Spec states:

The client MAY issue an HTTP GET to the MCP endpoint. This can be used to open an SSE stream, allowing the server to communicate to the client, without the client first sending data via HTTP POST.

Adding implementation to establish SSE connection through GET request. This is useful in particular for things like have tools/prompts/resource server notification to client - see examples.

Cleaning up README and adding more structure.

Remaining follow ups from Streamable HTTP transport

(mostly small changes, testing and cleaning up):

• Session management and Resumability (this is not small though )
• Refactor auth and make it work with session management
• Extended Examples for client and server
• Integration tests using examples
• Cancellation for streams - test and if any bugs fix
• Batching - mostly testing and checking if there are bugs
• Client to handle closing stream when all responses are done (bad server case)

[jspahrsummers]

Nothing blocking, I think, but there are some things probably worth fixing up before merge. 🙏