Optimize “bucket exists” checks

[sa7mon]

• Some providers like Scaleway return an x-amz-bucket-region header in the response to a HEAD request. In those cases, we can just make one request to any region.
• AWS and GCP definitely return the region after a single request. Make sure we’re not sending superfluous requests for each region client we’ve created while scanning.
• Switch providers to PathStyle, if they support it, to reduce DNS lookups.
• Measure the number of network requests (both HTTP and DNS) made before and after these changes

https://github.com/gaul/undocumented-s3-apis/