Set plumber options through env

User @slodge proposed to set plumber options through env variable.

Read an option value, if null, check for an ENV variable?

This would allow some of the API behaviors to change without redeploying code. I can see this to be useful in at least a couple scenario like activating debug with redeploying, changing secret.

Current user probably have some kind of options setting mechanism in place within their API, but it would be pretty "dope" to have it available before you know you need it.

I think this should be extended to all plumber options with a proper naming scheme like "PLUMBER_XXX_", replacing the dot in the option name with an underscore and having all caps?