proxy: Add ICE ephemeral ports range setting CLI flag

This MR add a CLI flag to set a range of ports that the ICE UDP connections can allocate from

The main usage of this is to allow NAT-ed networks to forward a limited range of ports to allow running the snowflake-proxy in an "unrestricted" operation mode (since the other way would be to open all the available UDP ports...)

A sample usage of this change is the following CLI invocation:

$ snowflake-proxy -verbose -ephemeral-ports-range 30000:40000

This would allow forwarding the 30000-40000 ports range on the router to get a working "unrestricted" Snowflake Proxy

PS: I'm a novice in Golang development (i'm more of a Rust person 😄), so i'm open to code improvement suggestions 😃