Add Headscale as "alternative" to Tailscale

it's rather a FOSS implementation of Tailscale server

Author: tituspijean

apps.toml

@@ -1308,6 +1308,7 @@ url = "https://github.com/YunoHost-Apps/headphones_ynh"
 [headscale]
 category = "system_tools"
 level = 7
+potential_alternative_to = [ "Tailscale" ]
 state = "working"
 subtags = [ "network" ]
 url = "https://github.com/YunoHost-Apps/headscale_ynh"