[verification] Return handle in client to make more ergonomic

[jhand2]

We should consider returning (*ContextHandle, <response_struct>, error) from most of the DPE client functions. Because DPE handles are single-use, always returning the new context handle makes the API easier to use.

Callers can use this unconditionally since even if using the default context, it is safe to reset to the default context each time.