Allow an integration to provide a step context initialization function

[aiwilliams]

I'd like the steps to receive an integration-specific sub-type of the IntegrationStepExectionContext, which has integration-specific properties on it used across the steps. This should happen once before the steps are called, and the executionHandler should be typed to receive the integration-specific context.

A concrete use case: initializing a single provider API client object. Without this mechanism, each integration has to invent a way to share a single instance of an API client across the steps.