Allow dedicated client-specific tests

[kegsay]

The tooling in complement-crypto is useful enough that it's often easier to write end-to-end tests in it than client-specific integration tests (e.g rust SDK). Unfortunately, the current structure/design has worked hard to make the tests agnostic to the underlying client implementation, but there is value in allowing tests to test client-specific things. We effectively already do this in places (e.g the notification tests which are very rust SDK focused), but we just kind of awkwardly add functions to the generic client interface.

Ideally, we should just let tests type-cast to a concrete impl. Considerations though:

• the impl may be behind an RPC barrier, so really we want to cast to a more specific interface than a real concrete impl,
• the specific impl/interface may not exist if it hasn't been built with the right -tags.