Add option to start audio-only call

[alya]

In the webapp, we have added the ability to start an audio-only call. We should do this in the mobile app as well.

TBD: Whether this feature will be implemented here, or only in the new Flutter app.

Related issues:

• Add option for disabling webcam by default when using jitsi zulip#12207
• Add Zoom support for audio-only calls  zulip#26549
• Add BigBlueButton support for audio-only calls zulip#26550