Rationale:
SipIceTranport was not standard-layout but a reinterpret_cast was called
on one member to retrive the class.
This "work" on Linux/gcc implementation but it's an undefined behavior
and not portable.
This changes the internal destroy transport callback:
now PJSIP is the unique owner of the SipIceTransport instance and
should call the destroy callback to delete the instance.
By the way, we remove shutdown and destroy method as not more used.

Side effect is the re-write of SipTransportBroker on how it handles
instance of SipIceTransport.

Refs #64903

Change-Id: I58848eca55c421de1883e53793fd58b0e1581d59