Ability to download whole Directory from remote CAS

[illicitonion]

The Store currently has a function to copy a list of Digests from the local store to the remote one.

It also has the ability to download individual Digests from the remote store to the local one.

We need to add the ability to download a Directory, and its contents, recursively.

This should probably take the form of a function named something like ensure_local_has_recursive which loads the top-level Directory, and drives a queue of Digests to download using the relevant load_bytes_* functions.

[ity]

thanks for putting that branch together @dotordogh I started working off of that branch and hopefully should have this out for review soon.

[ity]

should be able to pick this back up on friday/monday (out for the next 3 days)

[ity]

... and getting back to this today!

[ity]

implemented in #6336