Application protocol for "Open in Talk Desktop" links

[ShGKme]

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Is your feature request related to a problem? Please describe.

Register application protocol to allow:

• Open the app
• Open a specific chat
• Join a call
• Decline a call

Describe the solution you'd like

Register an app protocol like nextcloudtalk://chat/:id.

Describe alternatives you've considered

Additional context

It would allow:

• Have a button "Open in Nextcloud Talk Desktop" on the Web UI
• Open Nextcloud Talk Desktop from the Nextcloud Client, for example, by the Icon, of from the call notification:
  

Originally mentioned in: #386

[ShGKme]

RFC - protocol name:

• talk://
• nextcloudtalk://
• nextcloud-talk://
• nctalk://

[ShGKme]

RFC - protocol schema, including multi-account support:

• app://talk.nextcloud.ltd/call/{token}
• app://user@email@talk.nextcloud.ltd/call/{token}
• ...

[neufeind]

Related issue in the talk-server (spreed) issues: nextcloud/spreed#13902