Warn on nil types for configs that aren't part of the DEFAULTS hash #3340
Description
Identify all config params such as :web_transactions_apdex that can exist in the @config hash without having an entry in the DEFAULTS hash. Then warn in NewRelic::Agent::Configuration::Manager#handle_nil_type when a key is in play that is not on that allowlist. Currently, we just permit any key and return the value.