[Proposal][5.4] Add Redis connection contract

[hskrasek]

In Laravel 5.4, the old Illuminate\Redis\Database class was replaced with Illuminate\Redis\RedisManager. This new class returns either a Illuminate\Redis\Connections\PredisConnection or a Illuminate\Redis\Connections\PhpRedisConnection, both of which extend Illuminate\Redis\Connections\Connection.

Unfortunately neither of those classes extend an interface that can be type hinted against, so I propose we either add a Illuminate\Contracts\Redis\Connections\Connection interface or a Illuminate\Redis\Connections\ConnectionInterface

Potential 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 = []);
}

[taylorotwell]

Generally fine with me if someone wants to PR something simple.