[EXPORT S3]: Support Non-standard AWS S3 endpoints

[nswamy]

Use case

Users who have S3 in non-standard AWS endpoints (gov-cloud or private AWS cloud) cannot export their data to S3. We need to update export code to support non-standard aws endpoints, we could probably do a HEAD call on the s3 endpoint to validate the existence of s3 bucket.

dgraph/x/minioclient.go

Line 89 in 9a5ce69

return nil, errors.Errorf("Invalid S3 endpoint %q", uri.Host)

The general AWS S3 bucket URL is s3://s3.<region>.amazonaws.com/<bucket-name> however for classified regions and when using custom endpoints the URL would be s3://s3.<region>.<endpoint>/<bucket-name>
more info on private endpoint here