feat: allow overriding the address advertised in Record-Route

[kneliba]

get_record_route always builds the URI from the first entry of transport_layer.get_addrs() and a proxy can't advertise anything other than its primary listener. It also can't patch it afterwards either as the RecordRoute is built and returned in one call

why is this a problem?

• a P-CSCF terminating IPSec listens on an unprotected port and a protected port
• Record-Route has to advertise the protected one, or in-dialog requests come back to the
  unprotected port and arrive outside the security association, leaking data that should be encrypted

summary of changes

• get_record_route takes an Option<SipAddr>
• None keeps current behaviour.
  • only in-tree caller (examples/proxy.rs) passes None
• test in test_endpoint.rs covers both paths; build, test and fmt pass

note

• breaking signature change on a pub fn, happy to add a separate get_record_route_with_addr instead if that's preferred.

Thanks for your time!