Haven MIMI hub

This host runs a MIMI hub for Haven: the provider side of draft-ietf-mimi-protocol, for cross-provider rooms.

• The provider directory is at /.well-known/mimi-protocol-directory. It is public.
• The protocol endpoints are on port 8443 and need a client certificate. A connection without one is refused during the TLS handshake.
• The hub is mimi-hubd from havenmessenger/interop, which also lists where it differs from the drafts (DIVERGENCES.md).