Add hook for env var validation during generation time

[rogerluan]

This is a nice-to-have: having a hook where users of this gem can write validations to their secrets (in Ruby) during generation time. This is useful if, for instance, you'd like to check that a specific env var shouldn't contain and empty string which would cause your app to crash, or it's expected to be a boolean but it's a string, or any other type of issue that could cause problems during runtime, could be avoided during code generation time.