internal/mcp: reinstate ClientSession

Add a logical client session type back to the SDK, to bring it closer to
the design. Also, remove 'ConnectionOptions', which is better
implemented using middleware, and reimplement logging as a
LoggingTransport middleware, which is also added to the design.

Change-Id: I908b4f837d513ffc60ae5b6171a31519cffafebc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/672795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

mcp/sse.go

@@ -229,7 +229,7 @@ func (h *SSEHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 
 	// TODO(hxjiang): getServer returns nil will panic.
 	server := h.getServer(req)
-	ss, err := server.Connect(req.Context(), transport, nil)
+	ss, err := server.Connect(req.Context(), transport)
 	if err != nil {
 		http.Error(w, "connection failed", http.StatusInternalServerError)
 		return