internal/mcp: add batching support

Update the jsonrpc2 MCP framer to support both incoming and outgoing
batches. In order to achieve this, we must correlate the framed Reader
and Writer, which is not explicitly supported by the jsonrpc2 API, but
does work. A note is left to revisit the Framer interface.

Change-Id: If060cb5822da067833db20d58f5f112a0528da91
Reviewed-on: https://go-review.googlesource.com/c/tools/+/667295
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/jsonrpc2/frame.go

@@ -39,6 +39,15 @@ type Writer interface {
 // Framer wraps low level byte readers and writers into jsonrpc2 message
 // readers and writers.
 // It is responsible for the framing and encoding of messages into wire form.
+//
+// TODO(rfindley): rethink the framer interface, as with JSONRPC2 batching
+// there is a need for Reader and Writer to be correlated, and while the
+// implementation of framing here allows that, it is not made explicit by the
+// interface.
+//
+// Perhaps a better interface would be
+//
+//	Frame(io.ReadWriteCloser) (Reader, Writer).
 type Framer interface {
 	// Reader wraps a byte reader into a message reader.
 	Reader(io.Reader) Reader