Unify all client/backend communication by letting it go through the client

Acceptance criteria:

• Client DBus auth API returns localhost instead of the server
• On this localhost address, client offers an HTTP API which is proxied to the backend
• Local protocol is plain HTTP
• The client handles all authentication and mTLS and HW crypto.
• It is possible to turn the internal auth manager off, so that a different service can provide the connection.

Feature branch: feature-unified_comms