Fix optimized S3->S3 copy when the buckets have different credentials

We have an optimized code path to do direct S3-level copy of blobs that need to be copied between two S3 Blob Providers (both in the old S3BinaryManager and the new S3BlobStore).

However this code assumes that both buckets are covered by the same S3 connection parameters and buckets, and will fail if the credentials or endpoints are different.