STS Endpoint Environment Variable

[ahmadalkhansa]

Hello, I'm using s3cmd to access the S3 compatible API with Rados Gateway. I have to authenticate with an external OIDC provider and perform STS with Rados Gateway using an access token. I found that s3cmd supports STS but the endpoint variable is hard coded to Amazon's STS endpoint. Seeing the advantage of having an environment variable, I have created a Pull Request #1402 with a small modification to the code. I hope that this would be useful for others. Any feedback would be appreciated.