Implements OAuth Client ID Metadata Documents support, allowing servers
to accept URL-based client_id values where the URL points to a JSON
metadata document. This eliminates the need for Dynamic Client Registration
(DCR) in many cases.
- Private _cimd.py module (can swap for SDK when available)
- SSRF protection for URL validation
- Size-limited fetch with TTL caching
- CIMD enabled by default (opt-out via cimd_enabled=False)
- Advertises support via client_id_metadata_document_supported metadata
- Client-side client_metadata_url parameter
