Make IRPC v3 optionally backwards compatible

Specifically, we want IRPC v3 to be able to serve old v2 clients. This
way we can ship parts IRPC v3 stack incrementally.

To that end, allow IRPC v3 to implement v2 behavior of
generateCertificateRequest and testMode.

Bug: 260920864
Test: atest VtsHalRemotelyProvisionedComponentTargetTest
Change-Id: I9e47697bd948c8fd6b82147165d0c67bdef9fbd3
3 files changed