[5.x] Allow adding of GraphQL mutations

[Skullsneeze]

This PR adds the ability to add GraphQL mutations via config to statamic.

Reason for this feature

When creating a headless application there are various cases where you might want to work with GraphQL mutations. One example could be form submissions.

How to use it

In your project update graphql.php to add mutations. For example:

'mutations' => [
    SubmitFormMutation::NAME => SubmitFormMutation::class
],

Your mutation can be defined in a class like this:

<?php

namespace App\GraphQL\Mutations;

use GraphQL\Type\Definition\Type;
use Rebing\GraphQL\Support\Mutation;

class SubmitFormMutation extends Mutation
{
    public const NAME = 'submitForm';

    protected $attributes = [
        'name' => self::NAME,
    ];

    public function type(): Type
    {
        return GraphQL::type(SubmitFormResponse::NAME);
    }

    public function args(): array
    {
        return [
            'input' => Type::string(),
        ];
    }

    public function resolve($root, $args)
    {
        return 'Mutations response!';
    }
}

This will then allow you to run a mutation query like this:

mutation SubmitForm($input: String = "") {
  submitForm(input: $input)
  }
}