Feature Request: support s3 endpoint specified in the secret

[starpit]

Right now, the endpoint can only be specified in the Dataset resource. It would be helpful if one could also specify it in the associated secret. We are trying to establish an operator/user wall of sorts. The user knows what bucket and prefix to use, but rarely knows the endpoint url. The operator has more insight into that part of the world.

This would also allow the data to float around the world, independent of a fixed application spec of what bucket/prefix to use.