Add @better-auth/ciba implementing OpenID Connect Client-Initiated
Backchannel Authentication (CIBA) with poll and ping delivery modes.
Uses the oauth-provider extension protocol to register the
urn:openid:params:grant-type:ciba grant type and contribute
backchannel_authentication_endpoint to AS metadata.
Includes agent_claims forwarding, authorization_details propagation,
and OAuth Client Attestation header passthrough to sendNotification.