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) - the host's
:client_id callback when configured, otherwise the identifier the
credentials carried (the Basic/body client_id or the assertion sub), so
it is never nil for a successful authentication - 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).