Update the VTS test for CSRv3 updates

Conform to the latest CDDL changes. Organize parsing to observe the
AuthenticatedRequest structure.

Return the deserialized CSR payload rather than the DICE chain keys
because it simplified the return types.  The return value is only used
by one VTS test that checks sequential CSRs consist of the same request.
The test was incomplete before and it now only looks as the CSR payload
whereas it previously only look at the DICE chain keys.

Bug: 250910137
Test: atest libkeymint_remote_prov_support_test librkp_factory_extraction_test
Test: atest VtsHalRemotelyProvisionedComponentTargetTest
Change-Id: I1ba2e0cec22e25312fb890923a4c93043e9046cd