Support auth using AssumeRoleWithWebIdentity for non-AWS S3-compatible implementations

[Muon]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

We're using object_store for S3 access. We're trying to get OIDC auth working in our application and want to have automated tests of that using MinIO as an S3-compatible backend. However, object_store hardcodes the STS endpoint URL to https://sts.{region}.amazonaws.com.

Describe the solution you'd like

I'd like to be able to configure the STS endpoint URL when I build the client.

Describe alternatives you've considered

I've considered hacking around it using a hosts entry and a proxy, but that would fail HTTPS certificate validation.

Additional context

[tustvold]

I don't see any issue with making this configurable