Introduce Async APIs in WopiValidator

[RobRol]

This is the first of three changes to the WOPI validator which will migrate us from using synchronous APIs for doing the validation to asynchronous ones.

• This first PR introduces the async APIs, and creates a new command line option to use them, but defaults to the legacy path which behaves as it did previously.
• The next PR will flip the default behavior. This will migrate consumers to the async APIs by default, but they can still explicitly opt-in the synchronous APIs.
• The final PR will drop the synchronous APIs entirely forcing consumers to use the newer async ones.