Add public interface to search for folders in multiple folders using the same filter

[poizan42]

This can speed things up significantly when having to search for lots of folders with the same name from multiple mailboxes (requires delegation rather than impersonation though), one use case is when setting up subscriptions from a lot of mailboxes.

This also needs to make FindFolderResponse public so the responses can be represented.

[davster]

Looks good - thanks for adding this

[poizan42]

Hmm what happened to the @azurecla bot? (I have ensured that the cla is signed now, but it wasn't at the time it was merged)