rgw multisite and metadata abstractions/backends

multisite's metadata sync relies on some abstractions in order to transfer metadata between zones in json format. it also requires that any time we write changes to a metadata object, we also write to the metadata log so other zones know when/what they need to sync the main abstraction is the RGWMetadataHandler, which we implement for each kind of metadata. so we have a RGWBucketInstanceMetadataHandler that knows how to json-encode/decode and read/write bucket instances, a RGWUserMetadataHandler for users, etc etc in addition to json and reading/writing, these handlers also have some important logic in them. for example, when RGWBucketInstanceMetadataHandler writes a new bucket instance we haven't seen before, it will also create and initialize its bucket index objects the 'archive zone' uses special handler wrappers like RGWArchiveBucketMetadataHandler and RGWArchiveBucketInstanceMetadataHandler to force-enable object versioning, preserve deleted buckets, etc in https://github.com/ceph/ceph/pull/28679, we made a lot of changes to move the actual rados reads/writes into 'metadata backends'. Yehuda did a good job documenting the motivations and design decisions in the PR description, so i'd encourage everyone to read through that this predated the zipper work, but shared a similar goal of non-rados backends. however, this metadata backend stuff is complicated without providing any tangible benefits, while also making it significantly more difficult to add new types of metadata (see Abhishek's work to support role metadata in https://github.com/ceph/ceph/pull/37679). i'd like to see these backends reimagined in terms of zipper to allow metadata sync between different stores, but i think there are some open questions here: * zipper just has a Bucket interface - the distinction between the 'bucket entrypoint' and 'bucket instance' metadata is specific to the rados store * the split between MetadataHandler logic and the backends doesn't seem right, at least in the case of the RGWBucketInstanceMetadataHandler creating index objects, where the bucket index itself is a detail of the rados store i feel like a better approach would be for each zipper store to implement the MetadataHandlers itself instead of trying to maintain the separation between the metadata handlers and backends. so each store would have functions like create_bucket_metadata_handler(), create_user_metadata_handler(), etc then for the archive zone, its handlers would wrap the handlers it gets from zipper, and use generic zipper APIs to implement the extra stuff like enabling versioning