feat: Bi-directional bridging to external domains

This improvement of the B2BUA in `sip-bridge` mode enables the bridging
of calls from a Flexisip-managed domain to a third-party domain but also
from that third-party domain to the Flexisip domain, on a per-user level.

Provided that each user has an account on the third-party proxy, the
B2BUA is able to use that account to place a call to another user of
that third-party domain. That account will also be used when the B2BUA
receives a call from the third-party domain to recognise which Flexisip
user it was intended to, and bridge the call to that user.

To support that use-case, the `sip-bridge` JSON configuration now
supports more parameters, allowing finer control of its behaviour.
The previous structure is still supported, and will be internally
translated to the new parameters, but deprecated.

Co-authored-by: Anthony Gauchy <anthony.gauchy@belledonne-communications.com>