refactor: unified host fs store

[DavSanchez]

What this PR does / why we need it

Introduces a new trait OpAMPDataStore and makes K8sStore and a new FileStore implement it for operations regarding OpAMP data (CRUD remote config, identifiers, read local config, etc).

This way many of the structures that were originally dedicated for each implementation (Storer, ConfigRepositoryXXX) and contained mostly identical behavior have been replaced with a unique implementation that is generic over the new trait.

I tried to remove the intermediate traits (InstanceIDStorer, InstanceIDGetter, ConfigRepository) so the cleanup was even more deep and was a net removal of code, but I encountered some unexpected issues, mainly that OpAMPDataStore is not easy to mock with mockall due to it being highly generic, and also the fact that I would need to touch even more unrelated code to do that cleanup. So I leave this for a subsequent PR that might probably involve manual implementations of OpAMPDataStore mocks. This PR could be merged separately from that change.

Special notes for your reviewer

The commits called replace XXXX with generic one and remove redundant XXX impls do the replacement and cleanup respectively, so they touch quite a bit of code (for example, many tests for the on host implementations were moved from the storer/file modules and placed in file_store instead, there might be redundant tests there!).

Just a heads up in case you want to review them separately!

Checklist

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