Add pubip to piactl get command

[MysticRyuujin]

Description
This changes adds pubip to piactl get CLI tool. It fixes #14

Checklist

• I have described below how to test my code and included unit tests where possible.
• I have cleaned up the code and made best efforts to match the surrounding coding style.
• I certify that I have the right to submit this contribution under the GPLv3 and alter as outlined in the Contributor License Agreement.

Testing Plan
I have built this change from source and verified that it returns the Public IP Address as displayed in the GUI

[JonathonH-PIA]

Nice work @MysticRyuujin ! Could you also hook up the appropriate change signal in ValuePrinter::connectValueSignals() (getcommand.cpp) so piactl monitor pubip works?

Also super minor - please tweak the description to sentence case, i.e. Current public IP address 😁

We have a 2.4.0 in flight right now but I'd like to get this in our next release after that! (Shouldn't be any conflicts)

[MysticRyuujin]

The reason I didn't add it to the ValuePrinter::connectValueSignals was that I couldn't determine what to hook it to...?

vpnIp hooks to &DaemonState::externalVpnIpChanged but what would pubIp hook to?

&DaemonState::externalIpChanged ??

[MysticRyuujin]

It looks like that should work, I found the definition of externalVpnIpChanged and there's a matching externalIpChanged 😄

[JonathonH-PIA]

Great work, merging for next release!