feat: add guest auth prompt component

[skjnldsv]

This component is aimed to standardise this a bit more.

• There is one being used in Files (for files requests)
• Another in Talk to join a conversation

As I'm currently adding a bit more visibility towards how you're seen as a guest on an instance, I thought it would be nice to create this.

Features:

• Creates a new getGuestUser method that looks similar to a normal user
• Creates a dedicated showGuestUserPrompt that spawns a dialog
• Add a way to monitor displayName change to so whatever you want (reload an avatar, or change other stuff...)

Screenshots

Folder share

Talk room

Code snippet example

// Watching for displayname change in vue component
data() {
	return {
		currentUser: getGuestUser(),
		displayName: getGuestUser().displayName,
	}
},

mounted() {
	this.currentUser.addEventListener('updateDisplayName', () => {
		this.displayName = getGuestUser().displayName || ''
	})
},

[skjnldsv]

@susnux would be nice to have the vue props being exported in the docs.
But it seems more complex that my knowledge of it. Any clue ?
Else i'll investigate

[susnux]

Please not another API package with UI components.

1. It increases bundle size a lot even if you do not use Vue for your app
2. It bounds an API only library to UI
3. More complex dependencies
4. This creates a circular dependency between nextcloud-vue and nextcloud-auth

Instead this probably should belong just to @nextcloud/vue for UI components or if you consider it a dialog maybe also to @nextcloud/dialogs but makes no sense to make another simple logic library a UI package.

[skjnldsv]

Ah, sure thing!! Wasn't aware this was such a bad pattern 👍

[skjnldsv]

Smaller extract without UI: #795