This issue tracks work on the Streamable HTTP transport:

• Add a StreamableServerTransport type.
• Add a StreamableClientTransport type.
• Support 'stateless' mode for server transport (not documented, but implicit: see Streamable HTTP - session management inconsistency modelcontextprotocol#282 (comment)).
• Add configurable event store for the streamable server transport (see TypeScript's 'EventStore').
• Support retryability/replayability for the streamable client transport.
• Support a single JSON response (application/json)