Streamable HTTP support

[findleyr]

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.

[samthanawalla]

I can take a look at "Support retryability/replayability for the streamable client transport."

[findleyr]

@samthanawalla sure, that would be great. I started thinking about persistence in https://go.dev/cl/682555, but haven't done anything toward retryability. I think there is some good prior art in other SDKs. For example:
https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/client/streamableHttp.ts#L51

I will work on server persistence APIs.