Add the myrest_model submodule that holds the model as configuration

Problem/Motivation

The scanner, the planner and the generator all need somewhere to put what they learned, and the resources need somewhere to read it from. Keeping that in the main module would force every site to carry the model layer even when it only consumes handwritten resources, and would tie the release of one to the other.

Proposed resolution

• Create modules/myrest_model inside the project — one project on drupal.org, independently installable submodules, the layout of commerce and search_api.
• Ship it with the info file, the services file, the permissions and an empty configuration schema; the configuration entities themselves are filed separately.
• Declare its dependency on the main module and nothing else — no scanner, no generator, so that a production site can install the model without the tooling.
• Add it to the CI pipeline: PHPCS, PHPStan at the level the main module runs, and its own test group.

Remaining tasks

Everything. Skeleton only; the configuration entities arrive later.