[CT-2028] A general solution for setting attributes in flag objects to consistent types

[MichelleArk]

In the flag consolidation work done in API-ification phase 1, we introduced a method convert_config that converts the warn_error_options param as provided from a user config to the appropriate WarnErrorOptions.

Given that we define which type a param should be in cli.params, we can implement this casting more generally based on the click-configured type of the param instead of hardcoding it. This should apply for other params and types as well, so that they are consistently represented between Flags and params (e.g. Path params)