feat(ENGDESK-26634): add custom headers to invite and answer messages

[DeividVeloso]

• add custom headers to invite and answer messages

📝 To Do

• All linters pass
• All tests pass
• Change documentation based on my changes

✋ Manual testing

1. navigate to packages/js
2. run yarn build and yarn link
3. go to a webrtc project example and run yarn link @telnyx/webrtc
4. go to call.newCall({ customHeaders: [ { name: 'X-INVITE-Call', value: '9999999', }, ]})
5. check if this customer header shows on telnyx.rtc_invite WS message.
6. go to call.answer({ customHeaders: [ { name: 'X-Answer-Call', value: '9999999', }, ], })
7. check if this customer header shows on telnyx.rtc_answer WS message.

🦊 Browser testing

Desktop

• Edge (latest)
• Chrome
• Firefox
• Safari

📸 Screenshots

Description	Screenshot
Desktop
usage.gif