Refactor RegistryBatcher to make it reusable

What does this MR do?

This MR is one of the pieces while working towards #338597 (closed).

It refactors Geo::RegistryBatcher to move most of the logic into Geo::BaseBatcher. BaseBatcher can be reused in the new VerifiablesUpdateWorker/VerifiablesUpdateService` that is being worked on in !69301 (merged)

The main changes are the following:

• rename registry_class and model_class to destination_class and source_class, to make them more generic
• move tests into shared examples
• use these shared examples in registry_consistency_service_spec.

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How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

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Security

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