The "id" field is assumed to always exist, and must have a valid value.
Set "id" to unknown when planning new resource changes to indicate that
it will be computed.
This adds some of the required shim functions to the schema package.
While this further bloats the already huge package, adding the helpers
here was significantly less disruptive than refactoring types into
separate packages to prevent import cycles.
The majority of tests here are directly adapted from existing schema
tests to provide as many known good values to the shims as possible.
Martin Atkins