ABR3: Backup persistence

[gsheasby]

Goals (and why): Persist backup metadata in the same way as in the internal backup product.

Implementation Description (bullets):

• Add ExternalBackupPersister
• Wiring to AtlasBackupService

Testing (What was existing testing like? What have you done to improve it?): Added ExternalBackupPersisterTest

Concerns (what feedback would you like?): Storing the immutable timestamp seems a little janky - it appears to be used during the restore process though, so it would be needed at least for back-compatibility.

Where should we start reviewing?: ExternalBackupPersister

Priority (whenever / two weeks / yesterday): today 🙏

[changelog-app]

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Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

The AtlasBackupService will now persist backup metadata in the same way as in the internal backup product.

Check the box to generate changelog(s)

• Generate changelog entry

[gmaretic]

This looks good, haven't double-checked that it matches internal logic. If it's not difficult, we could smoke test it (I guess this is the task you mentioned in standup?), but it's also ok to go ahead with this and do an integration test down the line

[gmaretic]

Let's add a todo to eventually stop doing this. We want it now for backwards compatibility, but if there is not a valid use case for it (maybe it's used to repair targeted sweep tables, haven't checked?), we should get rid of it when reasonable.