title: Attaching a PaymentMethod to a Customer within PaymentIntent
body: <p>I am using ngx-stripe (frontend) and stripe-php (backend) and trying to create a Subscription for a Customer</p>
  <p>I have successfully implemented a card element on the frontend, which calls stripe-php to create PaymentIntent. Before the PaymentIntent is created, I use the submitted information to create a Customer</p>
  <p>Checking the successful payment in Stripe I can see the created customer attached to the payment and the payment has a payment method but if I check the customer, it shows as no payment method attached so I cannot create a Subscription for the customer</p>
  <p>Create customer:</p>
  <pre><code>$this-&gt;stripeClient-&gt;customers-&gt;create([
          'name' =&gt; $customer-&gt;getName(),
          'description' =&gt; $customer-&gt;getDescription(),
          'email' =&gt; $customer-&gt;getEmail()
  ]);
  </code></pre>
  <p>Create payment intent:</p>
  <pre><code>$paymentIntent = $this-&gt;stripeClient-&gt;paymentIntents-&gt;create([
      'customer' =&gt; $customerId,
      'amount' =&gt; $amount,
      'currency' =&gt; $currency,
      'payment_method_types' =&gt; ['card']
  ]);
  </code></pre>
  <p>Which Stripe API methods(s) do I need to call to instruct Stripe to create a payment method for the customer using the submitted details?</p>