Add support for accessing channel endpoints

[nerg4l]

As we getting closer to release channel and channels endpoint this library should get support for it.

We should introduce payloads for handling the response object.

Proposals for the API

Flexible

Channel:

$params = ['info' => ['user_count', 'subscription_count']];
// Probably should accept this as well
// $params = ['info' => 'user_count'];
$payload = $client->channel($channel, $params);

Channels:

$params = ['filter_by_prefix' => ['presence-']];
// Probably should accept this as well
// $params = ['filter_by_prefix' => 'presence-'];
$payload = $client->channels($params);

Specific

Channel:

$info = ['user_count', 'subscription_count'];
// Probably should accept this as well
// $info = 'user_count';
$payload = $client->channel($channel, $info);

Channels:

$filterByPrefix = 'presence-';
$payload = $client->channels($filterByPrefix);

[nerg4l]

We decided to lean towards a more specific implementation.