Implement Brave VPN on macOS

[bsclifton]

Description

Part of parent issue #15804

The goal here would be to create an implementation in brave-core which handles VPN on macOS. Each OS can provide an OS specific implementation for a "VPN Service".

This would need the ability to:

• Create a VPN connection with the OS.
• Update a connection (if details change)
• Connect to the VPN
• Disconnect from the VPN
• Remove the VPN connection from the OS

We should be able to do an implementation similar to what was done on iOS. Also see files in https://github.com/brave/brave-ios/tree/development/Client/Frontend/BraveVPN (@iccub lead the implementation on iOS)

[simonhong]

Add BraveVPNConnectionManager implementation with Guardian's https://github.com/brave/GuardianConnect

[simonhong]

Whenever application tries to configure VPN configuration via NEVPNManager api, macOS asks to user with below popup.

If Don't Allow is clicked, vpn configuration process can't go further.
If Allow is clicked, user have to type password again via below popup.

After inserting proper password, vpn is connected.
This is resolved. I put wrong data as a password when inserting item into KeyChain.