Properly handle full and partial sync - Part 1

What does this MR do and why?

• Issue: Properly handle full and partial sync (#397702 - closed)
• Additionally, fixes: Workspaces: Do not re-attempt workspace deletio... (#402766 - closed)

Properly handle full and partial sync.

This is part 1, which introduces the timestamp fields to the schema and API, and adds related logic changes to ee/lib/remote_development/workspace_create_processor.rb and other files.

A subsequent MR will perform more refactoring and test coverages to cover all the cases in the associated issue.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.