Support AWS_ENDPOINT_URL_S3 in aws backend

[rajatgoel]

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

Support S3 specific endpoint env var - AWS_ENDPOINT_URL_S3 (similar to how we support AWS_ENDPOINT_URL_STS).

Describe the solution you'd like

If AWS_ENDPOINT_URL_S3 is set, use that as S3 endpoint with highest precedence.

Describe alternatives you've considered

For now I am setting AWS_ENDPOINT_URL but this means I can't use separate endpoint for different services.

[tustvold]

Is this a standard environment variable, e.g. supported by the AWS SDK?

[rajatgoel]

Yes, this is a standard aws env var - https://docs.aws.amazon.com/sdkref/latest/guide/feature-ss-endpoints.html

[rajatgoel]

We already support it for STS in this repo. So this request is to just extend this to S3.