feat(slonik): add support for custom sql factory class

[opichon]

In slonik package, make it easier to customize the SqlFactory class in the Service.
Instead of having to rewrite the getFactory method, s Service class now only has to define its sqlFactoryClass property

import MySqlFactory from "...";

class MyService extends BaseService {
  ...
  get sqlFactoryClass() {
    return MySqlFactory;
  }

I have tested this minimally in the website-build package, and it needs to be tested much more thoroughly. In particular, please check other projects to see if they already define a custom SqlFactory class.

[rameshlohala]

I have tested it in our app, working as expected.
Most of the model in different projects defined a custom SqlFactory (extending DefaultSqlFactory) rewriting getFactory method so this changes will not affect them. but devs can refactor the existing model services to adopt this as well as implement in new services.