Add types to overlays code

[squaremo]

Either add the types file, or rewrite in typescript.

[squaremo]

First target: use the generated classes when constructing resources, e.g., in generateConfigMap. This probably means pointing the tests at dist/, or compiling into src/. If rewriting in typescript, the latter is probably better -- just have to make sure make test does the compilation step first, and the tests point at modules in dist/.