feat: Add PlaybackGrant

[sarahcstringer]

Fixes

Add a PlaybackGrant.

Checklist

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[sarahcstringer]

I verified here that I could attach a PlaybackGrant to a token and was able to connect with it:

<?php
require __DIR__ . '/vendor/autoload.php';
use Twilio\Jwt\AccessToken;
use Twilio\Jwt\Grants\PlaybackGrant;

$playbackGrant = new PlaybackGrant();
$playbackGrant->setGrant(array('requestCredentials' =>  NULL, 'playbackUrl' =>  'https://...', 'playerStreamerSid' =>  'VJXXX'));

$token = new AccessToken(
    $accountSid,
    $key,
    $secret,
    3600
);
$token->addGrant($playbackGrant);
echo $token->toJWT();

[sarahcstringer]

@shwetha-manvinkurke updated, thank you for the review!