Add support for Many-To-Many operations

[spersico]

I believe that this could be done in a couple of ways:

• Through the support of n-depth nested resources, and nested resources operations:
  So if I have an author model, a book model and a authorsBook model relating both, I can just delete an authorsBook (explicitly) and that's it.
  Or for GET operations, I can get to the books through authorsBook.

• Tough more... blackboxy? magicky?, another aproach could be something like this:
  https://loopback.io/doc/en/lb3/HasAndBelongsToMany-relations.html
  Where the user wouldn't have direct access to the junction-table, and the operations would be transparent to the user.

[joelalejandro]

This is being researched in the @kurier/addon-many-to-many package.

[marcemira]

Alright, here's a built-in implementation of manyToMany relationships feature/many-to-many, myself and @Martinarbez are working on a project that implements Kurier, but it was lacking this critical feature, we'll be happy to receive commentaries 🙏🏻 and hopefully get this feature landed on the package.