Support for Headscale

[Erisa]

An option (TNSET_CONTROL_URL) for an alt control server e.g. Headscale, was added in #55 however there may still be some remaining issues resulting in a "HTTPS is not enabled in the admin panel" error.

Making this issue to track Headscale usage testing, will be closed when all works to my liking.

[Erisa]

I've added two new options you'll need to use in order for tclip to work with Headscale:

• TSNET_CONTROL_URL needs to be set to your Headscale control server URL.
• DISABLE_HTTPS=true needs to be set, due to Headscale not supporting Tailscale Serve.

If you don't use both those variables in that way, you will see failure with "HTTPS is not enabled in the admin panel".

A full list of configuration options is available here: https://github.com/tailscale-dev/tclip?tab=readme-ov-file#configuration

If you run into any issues, please set TSNET_VERBOSE=true and create a new Issue with relevant log lines.