Credentials and identity reader

[tmi]

Adding two goodies I'm needing right now:

• configurable credentials for s3. No longer relying on reading from env vars, now those can be explicitly passed. Both to fs instance, and to a global config holder. Not optimal rn, but an improvement
• IdentityReader -- pretty much ls without a read. Fits nicely to the design

[ampx-mg]

lgtm, thanks!