Finish the conversion logic from scim filter to ecto query

[rmoorman]

Due to some personal matters, development on this stalled a bit. So let's try to get the ball rolling again.

Currently, we have a scim filter parser that converts a given filter string into a struct representation. But to make it useful, we have to:

• provide a default ecto integration module
  • aim for one public function that can take such a struct and generate a query struct that can used with ecto (e.g. composed with other queries)
  • the function should take the struct and a struct or function that encodes the possible mappings between scim filter fields and ecto fields in some way; e.g. if attribute familyName is matched, use the value to do a q |> where([user: u], u.family_name == ^value)
• clean up the tests that showcase how it works and cover the main use-cases
• provide documentation on how to use it