Flow scoped DI services through pipeline

[brettsam]

This includes creating the function class with the DI services (if not static).

[brettsam]

Initial design points:

1. Add an IFunctionExecutionContextFactory with one method: FunctionExecutionContext Create(InvocationRequest request)
2. Create a DefaultFunctionExecutionFactory... as a constructor it should take an IServiceScopeFactory. This is used to create the scoped services.
3. On FunctionExecutionContext, add a property IServiceProvider InvocationServices {get;}. This will hold the scoped services for that function invocation.
4. In order to create the scoped services, we'll need to pass an IServiceScopeFactory into the DefaultFunctionExecutionFactory via DI -- and then pass that into the FunctionExecutionContext when it gets created. IServiceScopeFactory is the thing that lets us call CreateScope() -- which gives us the IServiceProvider we want to use with InvocationServices
5. So when we create a new context, you can create the scope and then assign that to InvocationServices.

Then we'll iterate from there :-)