Fixture: plc_bsky_labeler.json

A synthetic DID document representing a did:plc labeler with the fields
required for correct labeler operation:

  - #atproto verification method (repo signing key).
  - #atproto_label verification method (label signing key).
  - #atproto_labeler service (labeler endpoint).
  - #atproto_pds service (PDS endpoint).

Both verification methods reuse the k256 test multikey from multikey_k256.txt.
