Add locale/language variants to paid-media-creative datatype

## Background

The current `components/datatypes/paid-media/paid-media-creative.schema.json` models a creative with a single set of content fields — one `xdm:title`, `xdm:body`, `xdm:callToAction`, `xdm:destinationURL`, `xdm:assets`, etc.

In production we observe Meta delivering a single ad creative (same `creativeID`) with **different** asset IDs, images, body text, and headlines for different locales. The current schema cannot capture that without losing fidelity.

## Reference

* [Meta Marketing API – Multi-Language Ads](https://developers.facebook.com/docs/marketing-api/multi-language-ads/)
* [Meta `asset_feed_spec` asset customization rules (`language_label`)](https://developers.facebook.com/docs/marketing-api/ad-creative/asset-feed-spec/asset-customization-rules/)
* [Google Ads API – RSA customization](https://developers.google.com/google-ads/api/docs/ads/customize-responsive-search-ads)

## Proposed Change

Add an `xdm:localeVariants` array property to the `paidMediaCreative` definition. Each entry carries:

* `xdm:language` — BCP 47 language tag (`en-US`, `ar-AR`, `fr-FR`, …), pattern reused from existing XDM datatypes (`profile-preferences-details`, `content-component-details`).
* Locale-specific overrides for `xdm:title`, `xdm:body`, `xdm:callToAction`, `xdm:destinationURL`, `xdm:displayURL`, `xdm:trackingURLs`, `xdm:assets`.

Top-level fields stay as the default/fallback (matches Meta's behavior where assets without `language_label` are treated as free-form defaults). To avoid duplication, `creativeAsset` and `creativeTrackingURL` are extracted into shared sub-definitions referenced from both the top-level fields and each locale variant, per [CONTRIBUTING.md – Re-Use and Modularity](../blob/master/CONTRIBUTING.md#re-use-and-modularity).

A new `paid-media-creative.example.4.json` demonstrates English / Arabic / French variants of a single image creative.

## Related

Tracked internally in Adobe Jira AN-451211 (parent design ticket AN-450480).

Start with components/datatypes/paid-media/paid-media-creative.schema.json and compare the language patterns in profile-preferences-details and content-component-details. Read CONTRIBUTING.md's Re-Use and Modularity section, then add the localeVariants structure, shared creativeAsset and creativeTrackingURL definitions, and paid-media-creative.example.4.json; done means the example represents English, Arabic, and French variants without duplicating definitions.

Written by the indexing model from the issue text.