Improve PaymentIntent metadata and event handling in Payment Request Button gateway

Problem/Motivation

When creating a PaymentIntent via the Stripe Payment Request Button gateway, the module provides limited/default metadata and does not give third-party modules a reliable opportunity to inject or override metadata before the PaymentIntent is created.
This leads to inconsistencies compared to the standard commerce_stripe gateway, and makes it hard for sites to send rich, business-critical metadata to Stripe.

Steps to reproduce

Proposed resolution

1. Dispatch a specific event prior to PaymentIntent::create(), allowing consumers to add/override intent attributes (especially metadata), mirroring the pattern used by commerce_stripe:
   • Event: Drupal\commerce_stripe_payment_request_button\Event\PaymentRequestButtonEvents::PAYMENT_INTENT
   • Payload: a PaymentIntentEvent-like object with getIntentAttributes()/addIntentAttributes()
2. Keep a single place to build rich metadata: the existing custom event subscriber.
3. Make sure each flow dispatches the corresponding event before PaymentIntent::create().
4. For renewals, either reuse the same event path or set metadata at the subscription level as a fallback.

Benefits

1. Restores parity with the standard commerce_stripe gateway’s extensibility.
2. Enables sites to uniformly enrich metadata across all Stripe payment methods, including Payment Request Button (Apple Pay/Google Pay).
3. Keeps a sane baseline (order_id, order_items,...) even if no subscriber is present.