This PR enables developers to add AspNetCore-style middleware to the task and activity dispatcher components. This custom middleware executes after the dispatcher has loaded the activity/orchestration but before it runs the executor logic.

Adding middleware is as simple as this:

TaskHubWorker worker = ...;
worker.AddOrchestrationDispatcherMiddleware(async (context, next) =>
{
    // pre-execute...
    await next();
    // post-execute...
});

worker.AddActivityDispatcherMiddleware(async (context, next) =>
{
    // pre-execute...
    await next();
    // post-execute...
});

You can add as many delegates as you want and they will be executed in-order for each task/orchestration execution. The context object contains a property bag as well as a few useful items from the runtime, like the OrchestrationInstance, OrchestrationRuntimeState, etc, that can be shared across middleware components of the same type.

See the new test code for more examples.

Example scenarios include adding diagnostics, monitoring, metering, etc. into the dispatch pipeline.