mgw: Allow auditing speciall 'null' endpoint

This is a special endpoint which can always be audited. This is useful
for clients who wish to submit requests to osmo-mgw periodically to find
out whether the MGW is still reachable. This endpoint will be used by
libomso-mgcp-client as default target endpoint to implement such
feature.
This "null" Endpoint is osmo-mgw specific, not described in MGCP specs.

Related: SYS#6481
Change-Id: Ia409b16e9211e6261e2e0f21288544289d6f3733
6 files changed