Add composite identity support using RequestStore (v2)

What does this MR do and why?

This MR is an alternative solution to building support for composite identity, which is different from

• Draft: Implement shadow user PoC (!168559 - closed)
• Draft: Introduce composite identity to API (!171090 - closed)
• Draft: Implement GitLab auth identity concept (!169946 - closed)
• Feature: Composite Identity for Service Accounts (#506641 - closed)

It is built on top of SafeRequestStore, where the scoped user is stored separately from the primary identity.