Add tooling to debug and validate env vars

[Firehed]

Especially in a K8S environment, it would be helpful to add commands to identify and diagnose env var issues (not set, invalid casts, etc). Being able to integrate this into health checks can increase reliability and stability of releases.

[Firehed]

Note: a fair bit of this already exists when reading the container. In library-consumer-space, it's straightforward enough to

a) read the config files,
b) filter to EnvVarInterface,
c) filter !hasDefault(), and
d) get the underlying env name with getName()

The latter two APIs are marked as @internal so it's somewhat inappropriate, but mechanically possible.