Some ActivityPub servers refuse to embed remote objects into their own
output. This is because they are not the authoritative source for these
objects, and as such embedding them is always a waste of space. The
follow request and follow models contain a URI, so this can be used to
match them.
- Tries to avoid performing HTTP request if the keyId is an actor URI
- Likewise if the URI is a fragment URI on top of actor URI
- Resolves public key, returns owner if the owner links back to the key
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