scrubber: perform manifest check with etag and refactors

[arpad-m]

This adds the etag to remote_storage::ListingObject, and uses it in the scrubber for performing the manifest freshness check added in #10007. This is in response to review giving feedback about the clock precision not being enough to prevent races.

I think the addition of the etag is useful enough as a general change, even without the motivation of wanting to do cheap checks whether there has been a change or not.

Note that the etag check still isn't 100% perfect, and we could get there also by just comparing the manifest, but I think the addition of the etag is useful enough of its own right. I don't know, please write if you think that it's baggage.

As for why the etag check is not perfect: it is fully determined by the content at least on AWS (md5 on AWS, didn't check Azure). So if you change a file from A to B and back to A, the etag will be the original one. Maybe something based on the version ID would be better, but we can't assume to always have it, as sometimes versioning is disabled on the bucket.

Follow-up of #10007

[arpad-m]

@problame do you think the ListingObject etag addition is worth it? If not, I'll close this, or remove it and just derive Eq on RemoteTenantManifestInfo.

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6e6eac6 at 2024-12-12T00:34:38.778Z :recycle:}