Allow payment intent to be altered to support free trials on recurring orders

This patch needs work, but don't want to spend a bunch of time on it if others think that we should go a different route (for example, using an Event vs alter hook).

An example implementation using this hook:

<?php

/**
 * Implements hook_commerce_stripe_payment_intent_create_alter().
 */
function hook_commerce_stripe_payment_intent_create_alter(&$intent_array, $order, $payment = NULL) {
  // Allow a free order to process.
  if (!$intent_array['amount']) {
    // Force manual capture - a free order shouldn't automatically be charged.
    $intent_array['capture_method'] = 'manual';
    // Get the price before discounts as the authorization amount.
    $intent_array['amount'] = \Drupal::service('commerce_price.minor_units_converter')->toMinorUnits($order->getSubtotalPrice());
    // $.50 is the minimum amount required by Stripe.
    if (!$intent_array['amount']) {
      $intent_array['amount'] = 50;
    }
  }
}

I think this is a duplicate of ✨ Allow customising payment intent attributes and metadata Fixed which just got committed. If it is not, please explain why.