Could just add mustEnv(X) as default, but we don't want secrets listed in help.
Could generalize other config sources:
CLI flag always wins, so
After parsing flags and before flagging missing requireds
We check environment, config files, config services etc with async checkConfig(key) => string => parse
Maybe enforce that required are the only things that have defaults to reinforce the semantic that optional means exactly "this param is an optional prop on the flags"
Could just add mustEnv(X) as default, but we don't want secrets listed in help.
Could generalize other config sources: