s3: Fix precondition failed in CopyObjectPart when src is encrypted (#7276)

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

cmd/object-api-errors.go

@@ -383,3 +383,15 @@ func isErrObjectNotFound(err error) bool {
 	_, ok := err.(ObjectNotFound)
 	return ok
 }
+
+// PreConditionFailed - Check if copy precondition failed
+type PreConditionFailed struct{}
+
+func (e PreConditionFailed) Error() string {
+	return "At least one of the pre-conditions you specified did not hold"
+}
+
+func isErrPreconditionFailed(err error) bool {
+	_, ok := err.(PreConditionFailed)
+	return ok
+}