Add caching to speed up pilot operations

[NickolausDS]

pilot list, pilot describe and other commands which search the remote filesystem can be slow due to needing to check various things before committing operations. This could be sped up drastically if we kept a local cache of search results and directory structure.

The cache should automatically go stale after a certain period of time. We may also want to set options to modify these behaviors, such as pilot project --clear-cache and pilot profile --no-cache to disable caching entirely.

I've done an implementation of this before for the petreldata.net portal here, however that only caches the projects config, not the directory structure of the remote endpoint.