Customize captures with before callbacks and driver context

[stevebauman]

I've added before() so you can customize a capture without creating a custom driver. Callbacks receive a driver-specific context implementing Contracts\CaptureContext. Its driver() method returns the actual driver instance, so you can use match or instanceof when the customization depends on the driver.

The Chrome context exposes page() for the loaded browser page before image bounds are measured or PDF preparation begins. The Gotenberg context exposes request() for the prepared request before it's sent, with a set() method for multipart fields and access to its URL and Guzzle options.

Custom drivers can implement the same context contract and expose their own resources. The hook is available on source, image, and PDF builders. Multiple callbacks run in registration order, output builders retain independent callback lists, and callback exceptions propagate through the existing cleanup.

Validated with 319 unit and feature tests, real Chrome capture tests, and Pint. Gotenberg request customization is verified using Guzzle's mock handler; its live service tests remain for CI.

This touches the Chrome capture construction also changed in #6, so those changes may need reconciling depending on merge order.