Add handler discovery of MediatR shim interfaces

[XL1TTE]

Implements IRequestHandler and IRequestHandler<T, TResponse> shim interfaces allowing MediatR-style handlers without MediatR dependency.

New files:

• src/Wolverine/Shims/MediatR/IRequest.cs
• src/Wolverine/Shims/MediatR/IRequestHandler.cs
• src/Wolverine/Shims/MediatR/WolverineOptionsExtensions.cs
• src/Testing/ShimsTests/ShimsTests.csproj
• src/Testing/ShimsTests/MediatR/* (test files)

Modified:

• build/build.cs - Added ShimsTests target
• wolverine.sln - Added ShimsTests project

Key features:

• No runtime reflection - uses TypeQuery discovery like IWolverineHandler
• No MediatR dependency - pure shim interfaces
• Full code generation support

[XL1TTE]

The CustomizeHandlerDiscovery feature has been utilized, allowing Wolverine to generate its own handlers, which helps eliminate reflection calls at runtime (Now i get it, thanks to Jeremy). It would be cool if users could migrate to Wolverine without having to explicitly change namespaces from MediatR's to Wolverine's. However, I understand it's likely not an option to reference non-MIT libraries from a commercial point of view.

[jeremydmiller]

@XL1TTE I'm going to get this in pretty soon, but move things around just a bit