Options for local file storage

[zlmarshall]

A feature request raised in the workshop today: If folks have files stored locally, there are additional access modes we could offer:

1. filename — just give me the names of the files with no path, and I will put the path on them

2. path — they provide a path as a parameter and we have to discover the files within that path (https://docs.python.org/3/library/os.html#os.walk maybe?)

3. endpoint — they provide a rucio storage endpoint (conceptually, this is likely the same as a path) and we discover the files on that endpoint.

Maybe 2 and 3 should be merged, since local folks will know both the name of their endpoint as a posix path and the name of their endpoint as a Rucio RSE, and it'll be easier (and probably faster?) for us to os.walk rather than to rucio list-files?