Add a LocalChangeFetcher

[omarismail94]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #2103

Description
Clear and concise code change description.

The LocalChangeFetcher is an interface introduced to streamline the process of fetching and iterating through localChanges, possibly sourced from databases, APIs, or other asynchronous sources.

The objective of the interface are:

• Abstract the logic of selecting local changes into a separate class
• Providing a flexible mechanism to choose different fetching strategies

To cater to various fetching requirements, an enum named FetchStrategy will be introduced. This enum will define all possible fetching strategies, such as fetching one local change at a time, or all the resources at once

Alternative(s) considered
Have you considered any alternatives? And if so, why have you chosen the approach in this PR?

Type
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Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct.
• I have read the Contributing page.
• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project.
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally.
• I have built and run the demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).

[jingtang10]

updated title after discussion

[jingtang10]

thanks omar - this is great.

only blocking change is in fhir engine impl. other comments optional and can create follow ups.