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>

examples/hello/main.go

@@ -50,8 +50,8 @@ func main() {
 		})
 		http.ListenAndServe(*httpAddr, handler)
 	} else {
-		opts := &mcp.ConnectionOptions{Logger: os.Stderr}
-		if err := server.Run(context.Background(), mcp.NewStdIOTransport(), opts); err != nil {
+		t := mcp.NewLoggingTransport(mcp.NewStdIOTransport(), os.Stderr)
+		if err := server.Run(context.Background(), t); err != nil {
 			fmt.Fprintf(os.Stderr, "Server failed: %v", err)
 		}
 	}