Consider creating a `CodegenMetadataSchema`

[karlvr]

In content.ts we create the part schema with a METADATA purpose, but the metadata schemas are quite particular and require quite particular handling in our generator templates. They have a value property which we need to access (see a901112) and perhaps header properties as well that require specific support.

A CodegenMetadataSchema type could contain the relevant info and signal to generator templates that special handling is required.