Codegen refactor required

1. Adding a new datamodel API to the codegen code requires specifying it in approximately five locations. We could define these specifications once in a declarative format along with metadata. This data could then be processed in multiple places.
2. There is a significant amount of boilerplate code in codegen. This boilerplate code can be significantly reduced.
3. I temporarily altered the top-level codegen script to generate code for a specific API. The script should be configurable via command-line arguments instead.

This refactoring will improve the maintainability, readability, and extensibility of the codegen mechanism, and it will also reduce the amount of code needed to add new datamodel APIs.

Start by locating the top-level codegen script and the approximately five locations where datamodel APIs are specified. Map the existing boilerplate and the temporary API-specific generation path before proposing a declarative format and command-line configuration. Done means API metadata has one source, boilerplate is reduced, and the generator supports selecting an API through command-line arguments.

Written by the indexing model from the issue text.