Add option to force P2P connection via WebRTC

[thinadusk]

In https://docs.netbird.io/about-netbird/how-netbird-works, it shows P2P traffic tunneling directly through WireGuard. If I understand correctly, this is raw, unencapsulated WireGuard traffic.

In some networks, WireGuard traffic may be restricted, and since Netbird already supports WebRTC for relaying, it would be helpful to add an option to use it for P2P connections.

This can be accomplished in a number of ways:

• Client-side environment variables
• Setting server-side ACL for some peers
• Global settings on the server

Related: #1096