Add optional WebDav propfind properties to count sub elements

[julien-nc]

This allows requesting {http://owncloud.org/ns}contained-folder-count and {http://owncloud.org/ns}contained-files-count PROPFIND properties which results in getting the number of files and directories contained by the element:

<nc:contained-folder-count>10</nc:contained-folder-count>
<nc:contained-file-count>58</nc:contained-file-count>

This can be useful in a folder tree view of a WebDav client to know in advance if a directory can be expanded without having to use depth=2 and make the Dav response potentially a lot bigger.

[PVince81]

@eneiluj once the spec is clear, please also add those to the documentation

[julien-nc]

@PVince81 Sure.
And if you have a better idea for the prop names, fine by me.

[juliusknorr]

How about contained-file-count and contained-folder-count for the naming?

[julien-nc]

@juliushaertl Yes, much better, thanks.

@PVince81 Documentation PR: nextcloud/documentation#8140

[julien-nc]

/backport to stable23

[julien-nc]

/backport to stable22