Amazon SNS transport. Publish-Subscribe support based on combination of SQS and SNQ

[makasim]

Amazon SNS transort implementation and a step towards #252 implementation.

<?php
use Enqueue\Sns\SnsConnectionFactory;

$factory = new SnsConnectionFactory([
    'key' => getenv('ENQUEUE_AWS__SQS__KEY'),
    'secret' => getenv('ENQUEUE_AWS__SQS__SECRET'),
    'region' => getenv('ENQUEUE_AWS__SQS__REGION'),
]);
$context = $factory->createContext();

$topic = $context->createTopic('test_enqueue');
$context->declareTopic($topic);

$message = $context->createMessage('a_body');

$context->createProducer()->send($topic, $message);

TODO

• docs -[sns][sqs] Add docs for newly added transports.  #769
• sqs integration

[makasim]

LGTM