internal/mcp: implement streamable HTTP transport

Add an (almost) complete implementation of the streamable HTTP
transport, including:
- A StreamableHTTPHandler for serving sessions.
- A StreamableHTTPServerTransport that supports session resumption.
- A minimal StreamableHTTPClientTransport.

Additionally, include the following minor changes:
- Refactor SSE event reading to use an iterator.
- Start to to aliases for JSON-RPC types (incomplete)
- Fix handling for null tool arguments (again)

Still to do:
- The streamable client does not support retry/resumption, or the
  side-channel GET requests.
- Support configurable session and event retention (garbage collection).
- Expose the HTTP client used for HTTP requests (for auth).

Change-Id: Ieba216ef901169b4660d601386fa0c7c7bb4c4b2
Reviewed-on: https://go-review.googlesource.com/c/tools/+/667515
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

design/design.md

@@ -58,6 +58,13 @@ Furthermore, the spec [states](https://modelcontextprotocol.io/specification/202
 Given the diversity of the transport implementations, they can be challenging to abstract. However, since JSON-RPC requires a bidirectional stream, we can use this to model the MCP transport abstraction:
 
 ```go
+type (
+	JSONRPCID       = jsonrpc2.ID
+	JSONRPCMessage  = jsonrpc2.Message
+	JSONRPCRequest  = jsonrpc2.Request
+	JSONRPCResponse = jsonrpc2.Response
+)
+
 // A Transport is used to create a bidirectional connection between MCP client
 // and server.
 type Transport interface {
@@ -204,7 +211,7 @@ func (*StreamableHTTPHandler) Close() error
 // that created the session. It is the caller's responsibility to delegate
 // requests to this session.
 type StreamableServerTransport struct { /* ... */ }
-func NewStreamableServerTransport(sessionID string, w http.ResponseWriter) *StreamableServerTransport
+func NewStreamableServerTransport(sessionID string) *StreamableServerTransport
 func (*StreamableServerTransport) ServeHTTP(w http.ResponseWriter, req *http.Request)
 func (*StreamableServerTransport) Connect(context.Context) (Stream, error)