Allow validation of the struct DTO or explicitly document unsupported types

[monco83]

Good day!

Autovalidation does not work when I try use it with struct DTO in MVC controllers.
The reason is here:
https://github.com/SharpGrip/FluentValidation.AutoValidation/blob/master/FluentValidation.AutoValidation.Mvc/src/Filters/FluentValidationAutoValidationActionFilter.cs#L64
and here:
https://github.com/SharpGrip/FluentValidation.AutoValidation/blob/master/FluentValidation.AutoValidation.Shared/src/Extensions/TypeExtensions.cs#L21

For some reason some types are not considered as 'Custom types' and ignored in the filter.

I suggest reconsider this logic and eliminate this validation completely or at least exclude from check list structs.
Honestly, I don't fully understand the reason for this behavior.

If you disagree to eliminate or mitigate this validation I can suggest:

1. customize this validation via options or specific interface injection,
2. at least describe this behavior in documentation.

[mvdgun]

Hi there, since this is a breaking change I put it in the v2.0 milestone. Should be an easy fix, will pick this one up ASAP. Honestly surprised this issue didn't pop up earlier.

[monco83]

Thank you