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.
• Support a single JSON response (application/json)

[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.

[samthanawalla]

We also need to support application/json #129

[jpmcb]

Support 'stateless' mode for server transport

I'd be happy to take a look at this since it's important for alot of servers I've been building.

Adding an option to ServerOptions to not expect session management seems like an ideal solution. This is how alot of other SDKs implement this (like in the Python SDK):

from mcp.server.fastmcp import FastMCP

# Stateless server (no session persistence)
mcp = FastMCP("StatelessServer", stateless_http=True)