Feature: Call transfer (blind and attended)

[deepakjaiswal1]

Summary

Add support for transferring an active call to another party.

Motivation

Call transfer is a core telephony feature missing from the current API. A softphone user needs to hand off an in-progress call to a third party.

Proposed capability

• Blind (unattended) transfer: transfer the active call to a target number without first speaking to the target.
• Attended (consultative) transfer: put the current call on hold, call the target, then connect the two.
• Define the outcome for the transferring party's own session after a successful transfer (released vs. retained).

Open questions for the WG

• New action/operation on webrtc-call-handling referencing the existing mediaSessionId, with a target identifier (phoneNumber)?
• How does the transferor authorize the transfer target, and what happens if the target rejects/does not answer?
• Notifications to affected parties via webrtc-events.
• Relationship to the 3-way calling proposal (attended transfer and merge share a "consult then connect" flow) — worth aligning the two designs.

@stroncoso @teikuran