Sign the requesting admin’s SSH public key via Vault.
If a publicKey is provided in the body, it is signed directly.
Otherwise the admin’s DB-stored SSH public key is used.
Bearer authentication header of the form Bearer <token>, where <token> is your auth token.
Optional request body for the admin certificate endpoint.
If public_key is provided the backend will sign it directly; otherwise
it falls back to the admin's DB-stored SSH public key.