Add: generate servers.description & servers.variables

[sun-yryr]

This PR enables the generation of server configurations that conform to the openapi specification.
Here is the reference I used.

Example:

config/openapi.php
~~~
'servers' => [
    [
        'url' => env('APP_URL'),
        'description' => null,
        'variables' => [
            'variable_name_A' => [
                'default' => 'default_A',
                'description' => 'sample',
                'enum' => ['A', 'B'],
            ],
            'variable_name_B' => [
                'default' => 'default_B',
                'description' => 'sample',
            ],
        ],
    ],
],
~~~

output json

~~~
"servers": [
    {
        "url": "http://localhost",
        "description": "",
        "variables": {
            "variable_name_A": {
                "description": "sample",
                "default": "default_A",
                "enum": [
                    "A",
                    "B"
                ]
            },
            "variable_name_B": {
                "description": "sample",
                "default": "default_B"
            }
        }
    }
],
~~~

[borisson]

Not sure if it is appropriate for me to review this, but I found some things that could be improved.

[vyuldashev]

Released under v1.3.0. Thanks for the contribution!