Extract cache migration code out of store.ts

[samayer12]

Describe what should be investigated or refactored

Related to #959 and #1323. store.ts has some hard-to-test code for migrating values when updating the store cache. Find a way to extract the migration logic that's part of PeprControllerStore.#migrateAndSetupWatch() into an isolated function that can be maintained independently.

Start this issue after #1259 is merged.

Links to any relevant code

Reference branch 1324-migration-refactor-reference for a starting point. Examine storeCache.ts and storeCache.test.ts to start out.

Additional context

You'll need to examine the type definitions and manage differences between cache entries of type Record<string, Operation> and DataStore entries which are defined as Record<string, string>.