Allow specific service typings for Hook and HookContext

[deskoh]

The service property of HookContext is of type Service<T>.

export default (options = {}): Hook => {
  return async (context: HookContext<User>) => {
    // No typings for `_create` method (catch-all definition will type unknown fields to `any)
    context.service._create(...)
    return context;
  };
}

This PR is backwards compatible and will allow the following:

import { Service } from 'feathers-mongoose';

export default (options = {}): Hook<User, Service<User>> => {
  return async (context) => {
    // Correct service typings.
    context.service._create(...)
    return context;
  };
}

[daffl]

I've been meaning to do this, thank you!