feat: add openapi URL support

It's helpful for openapi definitions to include
a server URL to send requests to (e.g. for aepcli
to use a single file to provide all required information).

example/bookstore/v1/bookstore.yaml

@@ -1,5 +1,6 @@
 # normally this would be suffixed with the domain (.com)
 name: "bookstore.example.com"
+url: "http://localhost:8081"
 resources:
   - kind: "Book"
     plural: "books"

example/bookstore/v1/bookstore_openapi.json

@@ -1,5 +1,10 @@
 {
   "swagger": "2.0",
+  "servers": [
+    {
+      "url": "http://localhost:8081"
+    }
+  ],
   "info": {
     "title": "bookstore.example.com",
     "version": "version not set"

example/service/service.go

@@ -71,6 +71,16 @@ func (BookstoreServer) GetBook(_ context.Context, r *bpb.GetBookRequest) (*bpb.B
 	return nil, status.Errorf(codes.NotFound, "book %q not found", r.Path)
 }
 
+func (BookstoreServer) ListBook(_ context.Context, r *bpb.ListBookRequest) (*bpb.ListBookResponse, error) {
+	var books []*bpb.Book
+	for _, book := range bookDatabase {
+		books = append(books, book)
+	}
+	return &bpb.ListBookResponse{
+		Results: books,
+	}, nil
+}
+
 func StartServer(targetPort int) {
 	bookDatabase = make(map[string]*bpb.Book)
 	lis, err := net.Listen("tcp", fmt.Sprintf(":%d", targetPort))