backend/s3: Add HTTP parameters for parity with AWS Provider

[gdavison]

The AWS Provider supports the following parameters. Add them for parity.

1. http_proxy. Also supports HTTP_PROXY and HTTPS_PROXY environment variables
2. insecure to allow insecure TLS requests. Defaults to false
3. use_dualstack_endpoint
4. use_fips_endpoint

[github-actions]

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