Add ability to join public calls without an account

To get a wider spread of Talk, and enhance the usability it would be nice to somehow generate automatic guest accounts that users could use with Talk.

So for example, if I choose a contact from the telephones contacts (full contact book) a link would be generated that automatically creates a guest user that are valid for let's say 1 hour.

That way anyone with Nextcloud and Talk installed could use Talk for private chats or video calls - not depending if they have Talk or not.

Maybe my idea isn't so thought through, but I would really like to make it easier for anyone using Talk without the need for a Nextcloud Server. I know that we have public conversations, but it would be nice to invite anyone using a guest account so that the external user could get the full benefit of using Talk on their Phone as well.

Maybe send something automatically generated per SMS like:

Hey! [Full Name] wants to talk to you on Nextcloud Talk. Download the app from Play here: [Link]
Your temporary credentials are:

Username:
Password: 

The credentials are valid for 1 hour.

The server address is: [Address of the senders server based on what's used for Talk in their config]

My proposal would need some work on the server side as well I guess.

Start by reviewing how Talk currently handles public conversations and invitations in the Android app. Clarify the server-side guest-account, temporary-credential, SMS, and one-hour validity requirements before identifying the Android entry points; done would mean a settled cross-project design rather than an isolated client change.