[5.6] Redis Connection Contract

[hskrasek]

This pull request aims to tackle laravel/ideas#399, but re-targeted to Laravel 5.6.

With this pull request we will gain a new Redis Connection interface, allowing developers to type hint against the interface instead of either A) hard coding to a specific connection type or B) not type hinting the connection parameter.

Currently Proposed Interface:

<?php

namespace Illuminate\Redis\Connections;

use Closure;

interface ConnectionInterface
{
    /**
     * Subscribe to a set of given channels for messages.
     *
     * @param  array|string  $channels
     * @param  \Closure  $callback
     * @return void
     */
    public function subscribe($channels, Closure $callback);

    /**
     * Subscribe to a set of given channels with wildcards.
     *
     * @param  array|string  $channels
     * @param  \Closure  $callback
     * @return void
     */
    public function psubscribe($channels, Closure $callback);

    /**
     * Run a command against the Redis database.
     *
     * @param  string  $method
     * @param  array   $parameters
     * @return mixed
     */
    public function command($method, array $parameters = []);
}

[tillkruss]

Can you please send this to 5.7 to avoid breaking changes, just in case?