migrate contender_cli to use contender_core::orchestrator::Contenter[Ctx]

**Is your feature request related to a problem? Please describe.**

`contender_cli` code to manage `contender_core` has essentially been rewritten; now the code in `contender_cli` is duplicative.

**Describe the solution you'd like**

Use `contender_core::orchestrator::Contenter[Ctx]` in cli instead of directly instantiating `TestScenario` and controlling it directly.

If there's any behavior that's not replicated in `orchestrator`, add it there.