Add IRequestValidator to more easily support injection (and mocking) of RequestValidator

[mducharm]

Issue Summary

The example for validating requests suggests that you disable validation in your tests, but I think it'd be nice to simply register an IRequestValidator in your app's service collection, allowing you to easily mock the validator when testing.

[AsabuHere]

This issue has been added to our internal backlog to be prioritised. Pull requests and +1s on the issue summary will help it move up the backlog.