Support some kind of var files

[Latios96]

Support some var files to author vars on the file level. Author them next to a cato config json file. They take precedence over vars in the config files, but cli vars can override them

One possible way could be to use .env files, which can be loaded with Python using python-dotenv. This way, environment variables could be used in general. Values for Cato should be prefixed with CATO. Be aware of CATO_API_TOKEN