Add support for `errors.As`

[roman-vanesyan]

• [x] I have looked at the documentation here first?
• [x] I have looked at the examples provided that may showcase my question here?

Package version eg. v9, v10:

v10

Issue, Question or Enhancement:

Provide support for the errors.As. Go 1.13+ has addition of errors.As that helps to cast given error the target if it or one of its nested errors implements the type of target.

To add support for the errors.As functionality all publicly available errors should implement As method of the following signature:

As(interface{}) bool

Code sample, to showcase or reproduce:

err := validator.Struct(&v)
if err != nil {
  err = fmt.Errorf("wrapped error: %w", err)
}

// ...

var validationErr *validation.ValidationErrors
ok := errors.As(err, &validationErr) // true