Add CODEOWNERS file with default team for all files (#343)

*Issue #, if available:* N/A

*Description of changes:* Adds a codeowners file which requires a review
from the S3 CSI Driver team before merging


By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

.github/CODEOWNERS

@@ -0,0 +1 @@
+* @awslabs/aws-s3-csi-driver