Expose `page.extra` field in the `@beacon` assign

The `@beacon` assign that is managed and build by the `BeaconAssigns` module holds public data about the current page as the page title, path, and so on. But currently it's missing the `extra` field which holds the data of [custom page fields](https://github.com/BeaconCMS/beacon/blob/main/guides/recipes/add-custom-page-fields.md). We want to expose that data because it's public and useful to build pages.

So when the `@beacon` is [created](https://github.com/BeaconCMS/beacon/blob/ce916b5c5f20e94737f3f6b54e1356ae525d6be9/lib/beacon/web/live/page_live.ex#L93) we want to have `@beacon.page.extra` with all the data of custom page fields available.

Start at lib/beacon/web/live/page_live.ex around the @beacon creation point and trace the BeaconAssigns module to see how current page data is assembled. Expose the page's custom-field data through @beacon.page.extra, then verify that the assign contains the expected values when a page has custom fields.

Written by the indexing model from the issue text.