Move DAV plugins to IBootstrap and make them independent of Sabre

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

Sabre plugins are registered through info.xml, therefore hard to discover, untyped and on top of that you have to use server's 3rdparty libs to implement one of the plugins.

https://github.com/search?l=XML&q=org%3Anextcloud+%22%3Csabre%3E%22&type=Code for all current usages.

Describe the solution you'd like

A proper OCP API that allows apps to register plugins in the IBootstrap::register method and doesn't require knowledge of any of the underlying tech stack, allowing easier Sabre upgrades.

Describe alternatives you've considered

N/a

Additional context

N/a

Start by reviewing the current registrations identified by the linked GitHub code search and the IBootstrap::register method mentioned in the issue. Map how apps currently register plugins, then define what the OCP API must expose so apps no longer depend on Sabre; done means registrations use that API and no longer require direct knowledge of the underlying stack.