scrubber: migrate tenant_snapshot to remote_storage

[arpad-m]

With #8673 and #8761 merged, there is only one remaining part of the scrubber that uses remote_storage: the tenant_snapshot feature.

This issue tracks migration of that last piece to remote_storage, which still uses the aws sdk.

I've talked about it with John and the plan was to first extend GenericRemoteStorage:

• addition of a DownloadOptions struct to the download function containing an optional and default off VersionId parameter. The struct can also be used for etag support, i.e. save downloading the file unless it actually changed (pageserver: optimize heatmap downloads with S3 conditional GET #6199)
• add a list_object_versions call that returns VersionId data

Then, those two functionalities can be used in the scrubber.

I'm breaking this out of #7547 as it is not the top priority.