AuthGraph: add per-role VTS tests

Add VTS tests that just exercise a single role, sink or source.

Use the AuthGraph core library to provide the implementation of the
other role.  This means that the tests are best written in Rust.

Put the test code into a library, so that other HALs which include
AuthGraph as a component can exercise the AuthGraph parts in their own
VTS tests.

Test: VtsAidlAuthGraphRoleTest
Bug: 284470121
Change-Id: I73f6aa277b41cc728587d707d7a6f82f0d18e08f