fix: #3620535 Support page variants

Canvas replaced page regions with page variants as the full-page layout a page is built from, but this module only knew about page regions, so variants were never offered: the route rejected the type and the listing never looked for them. Nothing failed loudly.

Page variants are added everywhere the layout types are listed: both controllers, both routing files and the client type union. Page regions stay, as the module supports Canvas versions from before the change.

The types are written as strings rather than class constants, since PageRegion and PageVariant do not both exist across those versions, and a layout type the installed Canvas does not provide is skipped when listing and answered with a 404 when requested.

Closes #3620535