Implement reusable operation signatures (operations as templates)

[daviwil]

It should be possible to use an existing operation as a template to define other operations, much like a model can be used as a template for other models. For example:

@autoRoute
@doc("Get a {name}", TResource)
@get
@readsResource(TResource)
op ResourceRead<TResource>(
  ...ResourceInstanceParameters<TResource>
): TResource | ErrorResponse;

model Widget {
  @key
  id: string;
  name: string;
}

op getWidget: ResourceRead<Widget>;

See the full approved proposal here: #217 (comment)