Add the myrest_scaffold submodule for the scanner, the generator and the administration UI

Problem/Motivation

The tooling that reads a site's model and writes PHP files is development-time code. It must be installable independently and, more to the point, uninstallable — a production site should be able to run the generated API without carrying a code generator.

Proposed resolution

• Create modules/myrest_scaffold with its info file, services, permissions and an administration route under /admin/config/services/myrest/scaffold.
• Depend on myrest_model; nothing depends on it.
• Declare a dedicated permission for scanning and generating, separate from the module's configuration permission — generating writes files to the filesystem.
• Add it to the pipeline like the model submodule.

Remaining tasks

Everything. Skeleton only.