Options for ConfigSources.defaultForTargets.
Optional
Allow the default app-config.yaml to be missing, in which case the source will not be created.
A custom substitution function that overrides the default one.
The substitution function handles syntax like ${MY_ENV_VAR} in configuration values. The default substitution will read the value from the environment and trim whitespace.
${MY_ENV_VAR}
Options for ConfigSources.defaultForTargets.