Moving credentials away from a strictly environmental concept

[rtyler]

I have been working on riverbank which has the need for specifying multiple buckets which may have multiple sets of authentication variables. At present, DeltaTable just assumes that the storage provider can figure out credentials on its own.

I think we need to change this to allow the caller to provide some form of a credential or credential provider, but I'm not sure how to accomplish this given the myriad of ways that credentials can be provider for AWS alone.

Coincidentally I saw this related issues this morning #278

[houqp]

I think it's a good time to promote new table method https://github.com/delta-io/delta-rs/blob/main/rust/src/delta.rs#L890 for more advanced usage. Relevant storage backend implementations need to be updated to support auth configuration in constructor methods.

[houqp]

While we revamp storage backend abstraction, might worth consider #257 as well.