CAS S3 backend endpoint validation

[hanygirgis]

Currently, adding a custom AWS-S3 CAS backend (with a custom endpoint) fails if the scheme is not part of the URL. For example:

$ chainloop cas-backend add aws-s3 --endpoint hostname:port ...

This fails with error "unexpected HTTP status code received from server: 504 (Gateway Timeout)"
However, it succeeds if a scheme is part of the endpoint (i.e. --endpoint https://hostname:port).
It might be a good idea to do validation of the endpoint and return a clearer message if it's invalid