mcp: make transports open structs

As described in #272, there is really no reason for transports to be
closed structs with constructors, since their state must be established
by a call to Connect. Making them open structs simplifies their APIs,
and means that all transports can be extended in the future: we don't
have to create empty Options structs just for the purpose of future
compatibility.

For now, the related constructors and options structs are simply
deprecated (with go:fix directives where possible). A future CL will
remove them prior to the v1.0.0 release.

For #272

internal/readme/client/client.go

@@ -20,7 +20,7 @@ func main() {
 	client := mcp.NewClient(&mcp.Implementation{Name: "mcp-client", Version: "v1.0.0"}, nil)
 
 	// Connect to a server over stdin/stdout
-	transport := mcp.NewCommandTransport(exec.Command("myserver"))
+	transport := &mcp.CommandTransport{Command: exec.Command("myserver")}
 	session, err := client.Connect(ctx, transport, nil)
 	if err != nil {
 		log.Fatal(err)

internal/readme/server/server.go

@@ -28,7 +28,7 @@ func main() {
 
 	mcp.AddTool(server, &mcp.Tool{Name: "greet", Description: "say hi"}, SayHi)
 	// Run the server over stdin/stdout, until the client disconnects
-	if err := server.Run(context.Background(), mcp.NewStdioTransport()); err != nil {
+	if err := server.Run(context.Background(), &mcp.StdioTransport{}); err != nil {
 		log.Fatal(err)
 	}
 }