Update ManagesCustomer to include optional metadata when creating or syncing a user

[dbhynds]

Our User model has the Billable trait applied to it. In order to make it easier to search Stripe for the correct user, we've been syncing the user ID in the metadata. I've noticed this has lead to a decent amount of code duplication.

class User extends Model
{
    public function refreshStripeData()
    {
        $options = [
            'metadata' => [
                'user_id' => $this->id,
            ],
        ];

        if ($this->hasStripeId()) {
            $this->syncStripeCustomerDetails($options);
        } else {
            $this->createAsStripeCustomer($options);
        }
        $this->updateDefaultPaymentMethodFromStripe();
    }

    public function ensureStripeUserExists()
    {

        if (!$this->hasStripeId()) {
            $this->createAsStripeCustomer([
                'metadata' => [
                    'user_id' => $this->id,
                ],
            ]);
        }
    }
}

This also means that if someone else is writing code that creates/syncs users in Stripe, they need to 1) know it won't send this metadata over and 2) remember to include it. It would be much easier if I could just define the metadata that should be included once on the User model and know that it would be included consistently every time.

This PR introduces a stripeMetadata() method to the ManagesCustomers concern. By default, it will just send over a null object and not set the metadata. However, if I wanted to specify some custom metadata on my Billable object, all I would need to do is add:

    public function stripeMetadata()
    {
        return [ 'user_id' => $this->id ];
    }

I haven't written any automated tests for this functionality, as I didn't see any for similar methods (e.g. stripeName(), stripeEmail(), etc.). I did manually test it and verified that it worked as expected for when no metadata is specified, and when custom metadata is provided.

[clementmas]

I'd like to see that PR merged so I could also replace:

$user->createOrGetStripeCustomer([
    'metadata' => $user->stripeMetadata()
]);

with

$user->createOrGetStripeCustomer();

[driesvints]

Looks good! Thanks