internal/mcp: change CallTool signature

CallTool takes a tool name, arguments, and an options struct.

Also, pass the params to ToolHandler.

Update the design doc accordingly.

Change-Id: I21b7995b607bea976ad708ebb6ac0716018f39cb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/672596
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/server_example_test.go

@@ -39,7 +39,7 @@ func ExampleServer() {
 		log.Fatal(err)
 	}
 
-	res, err := client.CallTool(ctx, "greet", map[string]any{"name": "user"})
+	res, err := client.CallTool(ctx, "greet", map[string]any{"name": "user"}, nil)
 	if err != nil {
 		log.Fatal(err)
 	}