Drop "@internal" annotation from the InputField class

[andrew-demb]

Since TheCodingMachine\GraphQLite\Middlewares\InputFieldMiddlewareInterface is allowed to be implemented by the userland and its method requires returning an instance of the TheCodingMachine\GraphQLite\InputField (or null) - I'm sure that the InputField class should not be marked with @internal tag - because there is no way to implement middleware without knowledge of the InputField class.

https://github.com/thecodingmachine/graphqlite/blob/5ed662c18a63ef7b5eed30bea27b82bd31477cab/src/InputField.php#L22-L30