feat: Add support for AWS_ENDPOINT_URL_S3 environment variable

[rajatgoel]

Which issue does this PR close?

Closes #589

What changes are included in this PR?

There are 2 changes -

• Support AWS_ENDPOINT_URL_S3 as an alternative env var that sets S3 endpoint.
• Process env vars in the same order as official AWS CLI/SDK does.

Are there any user-facing changes?

If this env var is not set, this should be a no-op. But if this env var is set,
our code will start respecting that. Probably worth calling out in the release notes.

[tustvold]

Unless this is something standard, i.e. supported by AWS SDKs, I'm a bit lukewarm on this. I don't want to be in a position where we have to support a huge number of config names just because they happen to collide in some people's environments.

with_config is the escape hatch for people who want to customise the environment variable logic

[rajatgoel]

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