Check a server's MSC1929 support file

Fetches and validates a server's MSC1929 support file (the contacts in /.well-known/matrix/support). Returns 204 when it is valid, or 400 with a list of the specific problems: empty file, a contact missing a role, an unsupported role, no contacts at all, or the deprecated 'admins' field. A handy pre-flight before relying on a server's support contacts.