Goodbye tunnels, hello agent endpoints

The ngrok team is introducing a change in terminology from "tunnels" to "agent endpoints", which aims to be more intuitive and align with industry standards. The agent configuration file now has a new version, v3.16.0, which introduces changes such as unified terminology and configuration across the CLI and agent configuration file, enhanced clarity and consistency, and traffic management handled by a traffic policy. Agent endpoint configurations are specified in the new `endpoints` field, with differences from the previous `tunnels` field including collapsed domain, scheme, and remote address fields into a single `url` field, as well as added flags for description and metadata. The ngrok CLI has also been updated to simplify endpoint network address specification and add new flags for description and metadata. A deprecation notice is in place for the old configuration file and deprecated CLI flags, which will be removed in a future release with a major agent version bump.