Transfer a SIP leg via REFER (asynchronous)

Asynchronously transfers a SIP leg. The HTTP call returns 202 as soon as the request is validated; the REFER is sent in the background and its outcome is surfaced through leg.transfer_initiated / leg.transfer_progress / leg.transfer_completed / leg.transfer_failed events. Blind transfer when replaces_leg_id is omitted; attended transfer when present (the named leg's dialog identity is embedded as a Replaces parameter per RFC 3891). On terminal 2xx the leg (and the replaces leg, if any) is hung up automatically.