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

examples/client/listfeatures/main.go

@@ -41,7 +41,7 @@ func main() {
 	ctx := context.Background()
 	cmd := exec.Command(args[0], args[1:]...)
 	client := mcp.NewClient(&mcp.Implementation{Name: "mcp-client", Version: "v1.0.0"}, nil)
-	cs, err := client.Connect(ctx, mcp.NewCommandTransport(cmd), nil)
+	cs, err := client.Connect(ctx, &mcp.CommandTransport{Command: cmd}, nil)
 	if err != nil {
 		log.Fatal(err)
 	}