Enable interception of model type used to fetch validators (for proxy type scenarios)

EditContextFluentValidationExtensions.GetValidatorForModel uses model.GetType() to determine what validator to fetch from the service provider. The majority of my view models are proxied at runtime and so model.GetType() will return the runtime-generated proxy type, not the underlying compile-time type being proxied. The result is that validators will sometimes not be found.

My proposal is to add a new parameter to EditContextFluentValidationExtensions.AddFluentValidation (in a non-breaking fashion).

public delegate Type InterceptEditContextModelTypeDelegate(object model);

public static EditContext AddFluentValidation(this EditContext editContext, IServiceProvider serviceProvider, bool disableAssemblyScanning, IValidator validator, FluentValidationValidator fluentValidationValidator)
	=> editContext.AddFluentValidation(serviceProvider, disableAssemblyScanning, validator, fluentValidationValidator, model => model.GetType());

public static EditContext AddFluentValidation(this EditContext editContext, IServiceProvider serviceProvider, bool disableAssemblyScanning, IValidator validator, FluentValidationValidator fluentValidationValidator, InterceptEditContextModelTypeDelegate modelTypeInterceptor)
{
	...
}

private static IValidator GetValidatorForModel(IServiceProvider serviceProvider, object model, bool disableAssemblyScanning, InterceptEditContextModelTypeDelegate modelTypeInterceptor)
{
	var modelType = modelTypeInterceptor.Invoke(model);
	...
}

I can submit a PR for this later.

[RyanMarcotte]

Documentation added to the existing PR

[Liero]

Actually, this scenario would be very well covered by #88 - ValidatorFactory callback which returns directly IValidator instance.

Advantage of ValidatorFactory is that it cover much more scenarios. For example you might used different validators for the same model type, you can use your own validator discovery logic, etc…

In fact, existing AssemblyScanning and ServiceProvider activation could be refactored as a ValidatorFactory callbacks.