Replace fog-aws with some implementation that uses the AWS SDK

Currently fog-aws re-implements a lot of what the AWS SDK does, but it doesn't support a lot of functionality, such as the ability to configure STS endpoints: https://docs.aws.amazon.com/sdkref/latest/guide/ss-endpoints-table.html

I've suggested in https://github.com/fog/fog-aws/pull/721#issuecomment-2315909148 that we replace the fog-aws CredentialFetcher with one that uses the AWS SDK: https://github.com/fog/fog-aws/blob/95b15032a746cac1f29c3bff3a6c758e9bd5c109/lib/fog/aws/credential_fetcher.rb#L49

In an ideal world we ditch fog-aws completely, but perhaps we could consider this transition plan:

1. Add support for STS URL endpoints in fog-aws: ✅ done in https://github.com/fog/fog-aws/pull/724 and !165708 (merged).
2. Add an optional credential fetcher option in fog-aws so we can pass in a fetcher that uses the AWS SDK. Possible, but perhaps not ideal: https://github.com/fog/fog-aws/issues/723#issuecomment-2336810001
3. Remove the need for fog-aws or re-implement a new fog-aws-sdk that uses the AWS SDK completely. Started prototyping on this; doesn't seem that bad.

ZD: https://gitlab-federal-support.zendesk.com/agent/tickets/9684