Expose HTTP API for managing Page Regions

Overview

Currently page regions are managed only through auto-save or direct config changes. To allow managing page regions through the CLI and Workbench (#3576837: [PP-1] Prototype managing the global layout (aka regions) within Canvas Workbench) we need HTTP API endpoints for getting, creating, and updating page regions.

Proposed resolution

Expose page regions on the canvas.api.config.list, canvas.api.config.get, canvas.api.config.post and canvas.api.config.patch routes. Deleting should not be allowed through the HTTP API (it's handled automatically on theme uninstall).

To avoid introducing the concept of a Drupal theme for a local code components codebase managed with Canvas CLI the list endpoint should include only regions for the default theme, and the post endpoint should allow omitting the region theme and assume then the default one.

User interface changes

n/a