✨(backend) add ServiceProvider

[qbey]

Purpose

We want to allow Service Providers to fetch our data but we also need a better management of the data available for each one of them.

Proposal

Not included in this PR: allow Service Provider management from the front.

⚠️ This disables the resource server globaly, to only allow it on a per-view basis because we don't want to allow any service provider to be able to query everything without filtering.

• [x] add the ServiceProvider model
• [x] allow Service Provider management from Django admin
• [x] restrict endpoint availability as a resource server
• [x] update changelog

[qbey]

@sampaccoud sorry I was not able to keep fixup commits as the refacto was too big to move resource server APIs to a dedicated module. I think I fixed all your comments (except for 404 vs 403, but we agreed IRL on this). The URL split for resource server makes the tests way cleaner :+1: and allows to have more specific endpoints, this is a good improvement :)