types(validation): support function for validator message property, and add support for accessing validator reason

[vkarpov15]

Fix #14496

Summary

The SchemaType.prototyp.validate API docs include an example of returning the original error message that the validator threw instead of Mongoose's automatic message formatting:

schema.path('name').validate({
  validator: function() { throw new Error('Oops!'); },
  // `errors['name']` will be "Oops!"
  message: function(props) { return props.reason.message; }
});

The TypeScript types disallow this currently.

Examples