HTTP API to read+write PageTemplate and Pattern config entities

Initial commit present, much left to be done!

The MR should now be green and supports the following operations for PageTemplate config entities, with detailed test coverage:

• listing all
• reading individual ones
• deleting individual ones — with a @todo for still needing to validate no other config entities depend on this one, see #3423459: [PP-4] JSON:API DELETE support for config entities.

Next up: POST and PATCH support, which is what will involve copying parts of #2300677: JSON:API POST/PATCH support for fully validatable config entities.

Per #4 and the green CI pipeline: ready for an initial review pass :)

(Tip: look at the diff locally, and exclude the first commit — which is literally just bringing in #3478537: Introduce an XB `PageTemplate` config entity.)

#3478537: Introduce an XB `PageTemplate` config entity landed. Merged upstream 0.x into this MR; this MR is now much smaller 👍

Fully ready, including detailed functional tests that prove config validation is doing its job 😊

BTW: one part I know still needs to happen: document the response shapes in /openapi.yml. But given my upcoming reduced availability, I think it'd be better for somebody else to take that on 😇

Reviewed by both @tedbow (from HTTP API POV) and @f.mazeikis (from config management POV), and previously by @longwave (from core committer POV).

All feedback addressed.

Let's land this :)