The configuration macro is changed to a registry that includes types
and default values next to the configuration names.

This is used for compile time generation instead of using runtime
checks for the configurations (environment variables).

For safety, failing to parse configurations is now going to log a
warning instead. This aligns with other platforms / tracers to not
crash a customer in production where their staging configuration
might be valid and their production one not.

The macro must now be used for any new configuration added and a CI
job verifies that all entries do exist. If a new env is added
otherwise it should fail.

The CI will also verify that the generated file is up to date and
matches the remote registry. It would fail, if either is not the
case.

The behavior change is documented in the README.md.