Expose available segmentation rules via HTTP internal API

Overview

For creating our personalization segments, we need an HTTP internal API to list which segment rules are available in the system.

Proposed resolution

• Provide a GET HTTP internal API that should provide the available segment rules.
• Provide a GET HTTP internal API that should provide the available settings (and their defaults) for a given segment rule.
• Ideally would work as the properties panel in components, so the Drupal form it's exposed and transformed in the client UI.
• Write kernel tests for these endpoints.
• Expand openapi specs

User interface changes

None.