Add REST APIs which consume our api.py

[MaferMazu]

Add REST APIs which consume our api.py.

• API methods needed
  • CRUD over the assignments (grants)
  • Method and endpoint to obtain all the users who have permissions over a resource.
  • Be able to filter and sort them
  • Method and endpoint to obtain the available roles, their description, and their permissions.

[mariajgrimaldi]

This was addressed by #84