Add Support for `ngrok`'s Agent `/endpoint` API

[alexdlaird]

In the latest ngrok Agent API, ngrok has now added parity support for /endpoints to align with /tunnels. This aligns with the v3 config files as well, so support should be added to utilize /endpoints instead of /tunnels when a v3 config is used (which should be the default going forward.

ngrok Agent API docs: https://ngrok.com/docs/agent/api#create-endpoint