Add a webfinger well-known handler for Nextcloud 21

[ChristophWurst]

• Resolves: n/a
• Target version: master

Summary

This adds the counterpart of the unfinished feature added via #954. Now the app registers a simple handler. The logic was simplified and moved into the handler. The helper class for the link was dropped because the new API has a typed method already.

• Requires Nextcloud 21
• Requires Add well known handlers API server#24702