Add hooks mechanism for extensible tracking

[sadjow]

This adds a hooks system so module consumers can extend the tracking pipeline with custom marketing parameters (e.g., fbclid, msclkid) or integrate with their own backends.

Three runtime hooks are now available:

• utm:before-track — decide whether to track the current page visit
• utm:before-persist — enrich or modify the collected data before it's saved
• utm:tracked — react after data is persisted (e.g., send to an API)

Hooks can be registered via a Nuxt plugin (nuxtApp.hook(...)) or through convenience methods on the useNuxtUTM() composable (onBeforeTrack, onBeforePersist, onTracked).

Also adds customParams to DataObject so hook consumers have a dedicated place to attach arbitrary data, and includes full integration test coverage for the hooks mechanism.