Fix compilation when generating both the middleware and orchestrator implementations (#2672)

## Motivation and Context
When setting the `enableNewSmithyRuntime` feature gate to
`both_default_middleware` or `both_default_orchestrator`, the generated
code would fail to compile due to the a checksum response decorator
assuming the property bag always existed. This PR makes that decorator
aware of the conditional existence of the property bag so that it can
elect to not customize the code for the orchestrator use case (which
will ultimately replace the decorator with an interceptor).

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