[Slim4] Add ref support to Data Mocker

Created by: ybelenko

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

Current progress

Now OpenApiDataMocker supports refs in array items key and object properties. When mocker cannot find referenced model it throws InvalidArgumentException.

<?php
require __DIR__ . '/vendor/autoload.php';

use OpenAPIServer\Mock\OpenApiDataMocker as Mocker;
$mocker = new Mocker();
$data = $mocker->mockFromSchema([
    'type' => 'object',
    'properties' => [
        'integer_from_1_to_100' => [
            'type' => 'integer',
            'minimum' => 1,
            'maximum' => 100,
        ],
        'float_from_minus3_to_3' => [
            'type' => 'number',
            'minimum' => -3,
            'maximum' => 3,
        ],
        'string_10chars' => [
            'type' => 'string',
            'minLength' => 10,
            'maxLength' => 10,
        ],
        'boolean' => [
            'type' => 'boolean',
        ],
        'array_of_strings' => [
            'type' => 'array',
            'items' => [
                'type' => 'string',
                'maxLength' => 20,
            ],
        ],
        'Object' => [
            'type' => 'object',
            'properties' => [
                'id' => [
                    'type' => 'integer',
                    'minimum' => 1,
                    'maximum' => 10
                ],
                'username' => [
                    'type' => 'string',
                    'maxLength' => 10,
                ],
            ],
        ],
        'Order_referenced_model' => [
            '$ref' => '#/components/schemas/Order',
        ],
    ],
]);

echo json_encode($data, JSON_PRETTY_PRINT);

output:

{
    "integer_from_1_to_100": 45,
    "float_from_minus3_to_3": -1.741,
    "string_10chars": "Lorem ipsu",
    "boolean": false,
    "array_of_strings": [
        "Lore"
    ],
    "Object": {
        "id": 3,
        "username": "L"
    },
    "Order_referenced_model": {
        "id": 1259632416,
        "petId": 350744286,
        "quantity": 331569927,
        "shipDate": "Lorem ipsum dolor sit amet, consectetur adipiscing el",
        "status": "approved",
        "complete": false
    }
}

TODO

Method mockFromRef should return model class instance(eg.new Animal()) not StdClass. As soon as Slim4 server has poor models implementation right now I will change it in future.

Related to #3545 (closed)

cc @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ackintosh, @renepardon