Add port forwarding with PIA using --port-forwarding

[BenLand100]

Disclaimer: I don't know rust, at all! [Seriously, check this code before doing anything with it!]

This PR removes --protonvpn-port-forwarding and makes a more generic framework triggered by --port-forwarding for API driven port forwarding that also works for PIA. In PIA this is done with an API instead of natpmpc, but I implemented the functionality in basically the same way as natpmpc.

This PR also has several limitations noted by FIXMEs relating to two points:

• Launching curl commands within the network namespace required the PIA certificate. I didn't know how to package this with the rust build framework, so hardcoded a local path on my machine for testing. Presumably this is a quick fix for someone knows rust.
• I didn't see an easy way to know the CN of the VPN vopono connected to. In openvpn this could be parsed from the config files (somehow), but wireguard was a bit more opaque. This method does work for both protocols. I've hardcoded the CN for the PIA Netherlands servers for testing.
• Ideally I'd like to specify a callback script to be invoked periodically, allowing the end user to do whatever needs to be done to spin up services on that port. If you're following so far, you won't be surprised that I hardcoded a path to a script here.

Hopefully someone can finish this off, or perhaps one day I may get more motivated to learn rust...

[BenLand100]

This has been a fun exercise playing with rust :) I'll add in a conf option for a callback script this afternoon sometime. Let me know if there are any other pre-merge changes to see to.

[jamesmcm]

Thanks overall it looks great, btw you should be able to run rustfmt with cargo fmt and clippy with cargo clippy locally to run those checks directly (and cargo fmt will reformat it directly).

I think the only thing left is to update the docs (README/USERGUIDE) on the usage too.