CopyObject precondition checks into GetObjectReader
in order to perform SSE-C pre-condition checks using the
last 32 bytes of encrypted ETag rather than the decrypted
ETag
This also necessitates moving precondition checks for
gateways to gateway layer rather than object handler check
if a bucket with `Captialized letters` is created, `InvalidBucketName` error
will be returned.
In the case of pre-existing buckets, it will be listed.
Fixes#6938
