internal/mcp: add tool and schema options

Implement variadic options for building tools and schemas, to allow
easier customization of tool input schemas.

Also, make fields required, unless they are marked as "omitempty".

Change-Id: I7a29258fae3ec5e7ea4906ed30ed6750979de962
Reviewed-on: https://go-review.googlesource.com/c/tools/+/668955
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

examples/hello/main.go

@@ -17,7 +17,7 @@ import (
 var httpAddr = flag.String("http", "", "if set, use SSE HTTP at this address, instead of stdin/stdout")
 
 type SayHiParams struct {
-	Name string `json:"name" mcp:"the name to say hi to"`
+	Name string `json:"name"`
 }
 
 func SayHi(ctx context.Context, cc *mcp.ClientConnection, params *SayHiParams) ([]mcp.Content, error) {
@@ -30,7 +30,9 @@ func main() {
 	flag.Parse()
 
 	server := mcp.NewServer("greeter", "v0.0.1", nil)
-	server.AddTools(mcp.MakeTool("greet", "say hi", SayHi))
+	server.AddTools(mcp.MakeTool("greet", "say hi", SayHi, mcp.Input(
+		mcp.Property("name", mcp.Description("the name to say hi to")),
+	)))
 
 	if *httpAddr != "" {
 		handler := mcp.NewSSEHandler(func(*http.Request) *mcp.Server {