Support for SIP proxy address.

[alfredjophy]

Issue

Currently, the address is used to set the host part for the sip address and to send the SIP invite. We have a case where the SIP server is accessed by a proxy, and the host part of the sip invite is the realm in the PBX system.

Fix

Adding a sip_proxy field to the CreateSIPOutboundTrunk call, and using it for sending the invite.

Would be open to contributing this feature.

[alexlivekit]

Does passing a Route header to send the request to proxy not work?