[Feature Request]: Support for providing (custom) SIP Headers for Cold transfer

[mohsen-parloa]

It'd be great to have this capability to provide a list of SIP headers as part of the REFER request.

example:

import { SipClient } from 'livekit-server-sdk';

// ...

async function transferParticipant(participant) {
  console.log("transfer participant initiated");

  const sipTransferOptions = {
    playDialtone: false,
    sipHeaders: {  // 👉 Here we provide a mapping of SIP headers
      "User-to-user": "00000",
      "X-Custom-Header": "ABCDE",
    }
  };

  const sipClient = new SipClient(process.env.LIVEKIT_URL,
                                  process.env.LIVEKIT_API_KEY,
                                  process.env.LIVEKIT_API_SECRET);

  const transferTo = "tel:+15105550100";

  await sipClient.transferSipParticipant('open-room', participant.identity, transferTo, sipTransferOptions);
}