Consider moving `TargetingHttpContextMiddleware` to the App Insights package

[rossgrambo]

We could move TargetingHttpContextMiddleware to the same package as TargetingTelemetryInitializer. The TargetingHttpContextMiddleware is not intended to be used without the TargetingTelemetryInitializer.

[jimmyca15]

If we do that, I'd like to see the library expose an

AddXyz

and

UseXyz pattern.

[jimmyca15]

I imagine that taking this change would result in changes in #416

[rossgrambo]

The middleware has been reworked to use Activity, which is no longer specific to App Insights. Leaving in the AspNetCore package now makes sense.