[Mastodon] Provide WebFinger backlink in AP actors

### Pitch In split domain setups (ActivityPub documents living on another domain than the one used in nicknames / WebFinger handles) discovery of this split-domain setup and validation both parties (the actor and the domain used in the nick handle) consent to the association can be a pain an brittle. Notably with just `preferredUsername` it’s not possible to detect whether the preferred nick handle domain changed later on requiring either redoing the discovery and verification process on each user update or simply accepting to miss it. FEP-2c59 makes all of this much simpler and more reliable by including the preferred handle in the ActivityPub actor data: like `"webfinger" => "user@example.org"`. It is both provided and used for verification and change detection by e.g. Akkoma. Currently Akkoma does redo the WebFinger discovery and verification on each update, but will soon switch to rely on `"webfinger"` and `"preferredUsername"` to only update when changes are detected. Providing