Implement listing favorites via the dav report API

[C0rby]

I added filter-files to the dav REPORT API. This enables the listing of
favorites.

This PR also includes a new in-memory storage.FS implementation which I use for the unit tests of the favorite package but it can also be extended and used for other unit tests depending on a storage.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[C0rby]

@butonic, could you have a look at this approach?

I think we still need some system to notify the several services about certain events like file created, file deleted, etc.
Here for example if a file is marked as favorite and then deleted, the favorite manager doesn't know that the files was deleted and will still return it.

Apart from that is that the expected approach? Do we still need to list all file from the storage provider and filter them for the favorite flag?

[labkode]

@C0rby @butonic
For every single operation we have to contact the favourite manager, which in a prod scenario it won't be in memory but a SQL or another storage that adds latency. I think this is a good starting point and I'm merging it as is to unblock the UI development.

However, this needs to be revisited. Either adding a cache or store this info in the web client and the web client will add the fav icon where needed by matching fileids from the webdav response.

[labkode]

@butonic @C0rby I've reverted this PR. I didn't realize of the hack of using the direct fs in the ocdav layer that bypass all the grpc logic, plus it breaks any deployment just by running it with another driver.

Please remove the dependency on the storage.FS, there is no reason to have it there and FS interactions can happen via GRPC.

[C0rby]

Right, I'll change the implementation to make the calls via GRPC and then we'll solve the potential performance problem later on.
Sorry for the inconvenience.