Change the orchestrator feature gate into a quad state (#2657)

## Motivation and Context
So far we have been testing with the orchestrator on or off, with "on"
meaning that both the middleware and orchestrator implementations exist,
and you have to opt into the orchestrator via a `send_v2` function call
(instead of `send`). This approach makes it difficult to test against
the orchestrator exclusively, and also test the existing integration
tests with the orchestrator implementation.

This PR changes `enableNewSmithyRuntime` into a quad-state with:
- middleware
- both middleware and orchestrator, defaulting to middleware
- both middleware and orchestrator, defaulting to orchestrator
- orchestrator

This allows for generating a client with the orchestrator exclusively,
or generating both and defaulting to the orchestrator, to reduce testing
friction.

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