Take account of AWS_ENDPOINT_URL in S3's is_url_valid method

[kingcrimsontianyu]

Currently syntax validation of an S3 URL relies on the url_has_aws_s3_http_format utility function, which only considers the actual AWS S3 URL formats (e.g. the virtual host style, the path style, etc). It should also take account of the environment variable AWS_ENDPOINT_URL to allow testing and support for other vendors (e.g. DigitalOcean Spaces, Backblaze B2, Wasabi, etc).

This limitation means that currently it is not possible to mock HTTP/HTTPS access to some of the endpoints, such as private/public S3 buckets, for the kvikio::RemoteHandle::open function.