This PR add support for ActivityPub (AP) identity verification. This allows all accounts on instances that support the AP protocol to be verified by the DOIP library.
Currently, some AP servers are supported, namely Mastodon and Pleroma. "Accidentally", other servers are also supported (like Pixelfed) but they are wrongly named as they were detected as being Mastodon or Pleroma.
This PR makes the following changes:
Add a new service provider, a new fetcher and a new API endpoint
Allow service providers to be searched for the proof in multiple locations