The authenticated client and how it authenticated.
:client is the opaque host client value returned by :load_client,
:client_id is the OAuth identifier (RFC 6749 §2.2) resolved through the
host's :client_id callback (or nil when none is configured), and
:method is the RFC 6749 §2.3 / OIDC Core §9 authentication method
(:client_secret_basic, :client_secret_post, :private_key_jwt, or
:none for the public-client path).