Support customised S3 servers endpoint URL
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Add support for customised S3 servers via checking the environment variable `S3_ENDPOINT`.

If set, the value of `S3_ENDPOINT` will be passed to the `endpoint_url` parameter for `boto3.Session`. It should be the complete URL to S3 service following the format: http(s)://{host}:{port}.

This feature is useful for companies who use their own S3 servers like MinIO, Ceph etc, as mentioned in issue  #26195