Add Support for ngrok's traffic_policy in Config

[alexdlaird]

ngrok is currently testing in preview a new config param traffic_policy. It looks like this param will exist in both the version 2 and v3 config files, and hopefully will look identical as objects, so that the implementation can be shared between them. We should wait to implement this until it is out of preview, but creating this ticket now as a placeholder.

• https://ngrok.com/docs/agent/config/v2/#http-configuration
• https://ngrok.com/docs/agent/config/v3/#endpoint-configuration-options
• https://ngrok.com/docs/http/traffic-policy/