rgw/notifications: support bucket notification with bucket policy

rgw/notifications: support bucket notification with bucket policy

following policy should be used to allow any user to get, put and delete
bucket notification on a bucket called "my-bucket":
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "Statement",
      "Effect": "Allow",
      "Principal": "*",
      "Action": ["s3:GetBucketNotification", "s3:PutBucketNotification"],
      "Resource": "arn:aws:s3:::my-bucket"
    }
  ]
}

note that notification deletion uses the "PUT" permission.

Fixes: https://tracker.ceph.com/issues/59136
Signed-off-by: Yuval Lifshitz <ylifshit@redhat.com>