[Slim] Abstract API controllers

Created by: ybelenko

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Inspired by #426 Generator produces abstract API controller classes which needs to be extended by implementation classes in ./src folder.

When implementation class doesn't exist application throws exception:

How about implementing getPetById as a GET method in OpenAPIServer\Api\PetApi class?

Upgrade note

Do not modify abstract API controllers directly! Instead extend them by implementation classes like:

// src/Api/PetApi.php

namespace OpenAPIServer\Api;

use OpenAPIServer\Api\AbstractPetApi;

class PetApi extends AbstractPetApi
{

    public function addPet($request, $response, $args)
    {
        // your implementation of addPet method here
    }
}

Place all your implementation classes in ./src folder accordingly. For instance, when abstract class located at ./lib/Api/AbstractPetApi.php you need to create implementation class at ./src/Api/PetApi.php.

Run $ composer dump-autoload every time your create new class.

To show error details with Slim modify index.php:

---$router = new SlimRouter();
+++$router = new SlimRouter(['settings' => ['displayErrorDetails' => true]]);

cc @jebentier @dkarlovi @mandrean @jfastnacht @ackintosh