A small collection of changes and code cleanup to successfully run
'corso repo init s3' manually. Adds a default s3 url (might want
this configurable for local testing) and aws session token support.
* add e2e wiring of cli to kopia
Now that pkg/storage and internal/kopia are in place, we
can wire up the init flow from the cli all the way to kopia. Testing
harness for this functionality still needs investigation afterward.
* factor out awsVars struct for s3Cfg
Adds the s3 provider set to the repo cli commands. Flag, env
vars, and other configuration is still incomplete and will require
iteration. But this sets us up for quick testing of the commands.
