Update validator registration mechanism

[ericproulx]

This PR replaces validator registration mechanism. Instead of relying on self.inherited, it registers validators by using const_get based on short name. Its does the same thing as the coercers.

Drawbacks

• custom validator must be declared in Grape::Validations::Validators modules and class's name must finish by Validator.

In our README, the AlphaNumeric example would need to be in declared like this:

module Grape
  module Validations
    module Validators
      class AlphaNumericValidator < Base
        def validate_param!(attr_name, params)
          unless params[attr_name] =~ /\A[[:alnum:]]+\z/
            raise Grape::Exceptions::Validation.new params: [@scope.full_name(attr_name)], message: 'must consist of alpha-numeric characters'
          end
        end
      end
    end
  end
end

Benefits

• Custom validators are declared the same way as the built-in ones.
• Less confusion about where to add custom validators. Following the convention would naturally lead to something like lib/grape/validations/validators/custom_validator.rb
• Registration done a when compiling instead of requiring

[grape-bot]

	1 Warning
⚠️	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#2508](https://github.com/ruby-grape/grape/pull/2508): Update validator registration mechanism - [@ericproulx](https://github.com/ericproulx).

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