Move convenience class into new file

[fixlr]

This PR moves the Grape::Types::InvalidValue class definition into a separate file so that it can be lazy loaded in projects that use zeitwerk file structure conventions.

In environments that do not eager load, referencing the Grape::Validations::Types::InvalidValue class before Grape::Types::InvalidValue causes a load error due to the fact that both classes were defined in the same file.

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Testing?! Yes! Kind of...

I haven't come up with a good way to write a new rspec example for this change, but one test did fail on my branch before I updated lib/grape.rb. This failure occurred in coerce_spec.rb:271 because Grape::Types::InvalidValue was no longer being autoloaded. (It raised the same uninitialized constant Grape::Types exception that you'll see in my example below.)

For now, in lieu of tests, I created a simple gist that demonstrates the issue I'm seeing in a private Rails application. Here are the results on both my branch and master:

> git checkout master
> ruby grape_autoload_bug_sample.rb
grape_autoload_bug_sample.rb:15:in `<main>': uninitialized constant Grape::Types (NameError)

puts "Loaded #{Grape::Types::InvalidValue.name}"
                           ^^^^^^^^^^^^^^ 

> ruby grape_autoload_bug_sample_reversed.rb
Loaded Grape::Validations::Types::InvalidValue
Loaded Grape::Types::InvalidValue

> git checkout extract-grape-types-invalid_type
> ruby grape_autoload_bug_sample.rb
Loaded Grape::Types::InvalidValue
Loaded Grape::Validations::Types::InvalidValue

[grape-bot]

	1 Warning
⚠️	There're library changes, but not tests. That's OK as long as you're refactoring existing code.

Generated by 🚫 Danger

[dblock]

Awesome, thank you. Let's merge this.

And thank you for trying to write a test for it. If you still feel motivated, I think you should be able to either extend https://github.com/ruby-grape/grape/tree/master/spec/integration/eager_load or make a new test that's similar. And if you feel even more extra motivated, we probably have multithreading bugs related to eager-loading, too.