Changed column type of timestamp in LocalChangeEntity

[aditya-07]

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

Fixes #2083

Description

1. LocalChangeEntity.timestamp is change from human readable string to Instant like in other tables in the engine's db.
2. Added Migration and tests.

Alternative(s) considered
NO

Type
Choose one: Code health

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).

[omarismail94]

Thanks for this! Just one comment

[jingtang10]

thanks aditya - great change!