mcp: new API for features (#88)

BREAKING CHANGE

Change the API for adding tools, prompts, resources and resource
templates to a server.

The ServerXXX types are gone along with the plural Add methods
(AddTools, AddPrompts etc.)
Instead there are single Add methods (AddTool, AddPrompt).
In addition, instead of NewServerTool, there is AddTool[In, Out].

Fixes #73.

mcp/server_example_test.go

@@ -29,7 +29,7 @@ func ExampleServer() {
 	clientTransport, serverTransport := mcp.NewInMemoryTransports()
 
 	server := mcp.NewServer("greeter", "v0.0.1", nil)
-	server.AddTools(mcp.NewServerTool("greet", "say hi", SayHi))
+	mcp.AddTool(server, &mcp.Tool{Name: "greet", Description: "say hi"}, SayHi)
 
 	serverSession, err := server.Connect(ctx, serverTransport)
 	if err != nil {