Cache information about the namespace

[bbockelm]

This PR adds a cache which tracks information about the various namespace prefixes.

On cache miss, the prefix and the list of endpoints are put into the cache (caching for up to 15 minutes). This information is reused on subsequent opens of the object.

Additionally, to ease my local testing, this registers the osdf:// and pelican:// schemes with fsspec, allowing this to be a valid mini-test:

import fsspec
with fsspec.open("osdf:///ospool/uc-shared/public/OSG-Staff/validation/test.txt") as of:
    print(of.read(1024))

[bbockelm]

Just to get things going here, I'm going to merge.

We now have unit tests! 🎉