[Meta] Add application protocol with with "Join conversation" handling

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Feature request

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

There must be a simple way to open an app from an invitation (like an email).

Describe the solution you'd like

1. Register application protocol nextcloudtalk:// following Web version schema:
   nextcloudtalk://[<userid>@]<server_host>/<server_base>/<path>[?query]:
   • nextcloudtalk://user1@cloud.tld/base/index.php/apps/spreed?callUser=user2
   • nextcloudtalk://user1@cloud.tld/base/index.php/call/abc123#direct_cal
   • nextcloudtalk://user1@cloud.tld/base/index.php/login/
2. Add an invitation web page (for example, https://cloud.tld/apps/spreed/invite/<token> or https://cloud.tld/call/<token>/invite)
   
3. Additionally, add a button in the conversation to quickly open it in the app like a popup on a mobile version
4. "Copy conversation link" should be an invitation link

Describe alternatives you've considered

Alternative protocol schema: nextcloudtalk://<action>?server&withUser&withRoomToken:

• nextcloudtalk://login?server&user
• nextcloudtalk://open-conversation?server&withUser&withRoomToken

Additional context

What if there are 2 apps installed with nextcloudtalk:// handler simultaneously? (double-check, please)

• Windows- the last registered app wins, but user can change it in the settings
• macOS - the last registered wins, no user settings
• Linux - depends on desktop environment
• iOS - user is prompted to choose
• Android - user is prompted to choose

What if a domain is enforced? In this case there must be no choice.

Probably, we need a server setting with the application protocol value.

No files, tests, or entry points are named. Start by reviewing the proposed nextcloudtalk:// schemas and invitation URL flows, then define how protocol registration, invitation handling, conversation links, and the open-in-app action should work across the listed platforms; done requires an agreed design and working behavior for those flows.