common, axum: use the #atproto verification method as the signing key #73

DidDocument::atproto_multikey and jacquard-axum's extract_signing_key took the first Multikey (axum: any first key). The DID spec (https://atproto.com/specs/did) says the signing key is the entry whose id ends in #atproto, typed Multikey and controlled by the DID, and that others are ignored. real example: did:web:offbeatwit.ch lists #atproto_old first, so its commits were verified against the old key and rejected.