rgw/s3: support paginated ListBuckets

rgw/s3: support paginated ListBuckets

at some point, aws added pagination support to ListBuckets[1] in the
form of query params max-buckets and continuation-token

RGWListBuckets already supported pagination for the swift api, so we
only need to hook up the s3 request/response formats to use it

Fixes: https://tracker.ceph.com/issues/72315
[1] https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListBuckets.html

Signed-off-by: Casey Bodley <cbodley@redhat.com>