AWS bucket notifications

Dear Community, Prateek Sachan has done a great job in adding AWS SNS and Lambda, as bucket notification destinations, as his GSoC project [1]. Work is ready for review here [2]. Any comments or feedback is much appreciated! Yuval [1] https://ceph.io/gsoc-2020/#cloud-endpoints-for-bucket-notifications [2] https://github.com/ceph/ceph/pull/36062/