Create a SIP trunk (REGISTER or static peering)

Creates a typed SIP trunk. For type: "sip_register", VoiceBlender begins REGISTERing to the supplied registrar URI with digest auth, refreshes before expiry, and routes inbound INVITEs that arrive on that peer's socket plus outbound INVITEs whose from matches the AOR through the trunk. Set sip_register.outbound_proxy to send both the REGISTER and those INVITEs via a next-hop proxy instead of straight at the registrar; the Request-URI and digest authentication still target registrar_uri. It defaults to SIP_OUTBOUND_PROXY and is resolved at creation time, so the trunk snapshot reports the hop actually in effect. For type: "ip_ip", VoiceBlender peers with a fixed upstream without registering: the trunk is active at once, outbound INVITEs placed through it (by trunk_id on POST /v1/legs, or by a from matching ip_ip.aor) are routed at ip_ip.peer_uri with the trunk's optional digest credentials, and inbound INVITEs whose source address falls in ip_ip.inbound_sources are tagged with the trunk. Set ip_ip.options_ping_interval_seconds to health-check the peer with OPTIONS and receive sip.trunk_up / sip.trunk_down events.