[11.x] Multiplan subscriptions

[driesvints]

This PR implements multiplan subscription support into Cashier.

Some examples:

// Add a new plan to a subscription...
$subscription->addPlan($plan);

// Add a new plan to a subscription and invoice immediately...
$subscription->addPlanAndInvoice($plan);

// Add a new plan with a specific quantity...
$subscription->addPlan($plan, $quantity);

// Remove a plan from a subscription...
$subscription->removePlan($plan);

// Update quantities for plans...
$subscription->updateQuantity($quantity, $plan);

Stripe docs: https://stripe.com/docs/billing/subscriptions/multiplan
Stripe API: https://stripe.com/docs/api/subscription_items

Closes #397

Todo

• Update swap and resume methods on Subscription model
• Update onTrial, subscribed, subscribedTo and onPlan methods on ManagesSubscriptions trait
• Maybe introduce easy way of applying tax rates to individual subscription items?
• Add new methods on Subscription model
• Write tests