annotation: Add support for multiple HTTP operations

[RussellLuo]

Sometimes, a Go method needs to correspond to more than one URI (or HTTP method). For example:

type Service interface {
	//kun:op GET /messages/{messageID}
	//kun:op GET /users/{userID}/messages/{messageID}
	GetMessage(ctx context.Context, userID string, messageID string) (text string, err error)
}

This will enable the following two alternative HTTP to Go-method mappings:

HTTP	Go Method
GET /messages/123456	GetMessage(ctx, "", "123456")
GET /users/me/messages/123456	GetMessage(ctx, "me", "123456")

This feature is much like (and is partially inspired by) gRPC's additional_bindings option (also see Multiple URI bindings).