chore: Add event to register mount providers more lazy

[juliushaertl]

Mount provider registration currently requires to obtain the MountProviderCollection and create instances of apps mount providers, which might pull in more heavy dependencies. While this is not a problem in files_sharing, other apps like circles or collectives have a larger tree of dependencies that the mount provider class is pulling in.

We don't need the mount providers always available so we can save those DI setup times for all requests that don't access the mount provider list through accessing the storage.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[come-nc]

An other solution would be to register provider classes and let the mountprovidercollection get them from DI when needed?

Only problem is if an app want to register instances with parameters in the constructor, or even several instances of the same class with different config. In this situation your solution works better.

[come-nc]

Suggested change

	$this->$this->registerEventEmitted = false;
	$this->registerEventEmitted = false;

I suppose?