Allow fields to be added to the page entity type

Overview

Discussed in this slack thread: https://drupal.slack.com/archives/C072JMEPUS1/p1768606306603599

It's a very common use case to add field UI fields to entities for non-content purposes. A use case already in the canvas issue queue is #3544662: Remove custom Metatag implementation where the metatags field has been added as a base field (and therefore not removable), but instead could be omitted and added by sites that needed.

Other cases are things like 'hide from search results' or fields that interact with custom access control. People can add base fields for those if they're using exclusively custom code, but a lot of things like metatags are not like that.

Proposed resolution

Add field_ui and any other support, might need some additional text on the field UI page that fields can't be configured to actually display.

User interface changes