Add support for Site-To-Site VPN Configuration

[DDxPlague]

Right now it looks like the unifi_network resource does not support creating a site-to-site VPN network. I'm not sure if this needs to be an extension to the unifi_network resource or if we should create a new resource entirely like unify_site_to_site_vpn.

My usecase: I'm standing up an AWS site-to-site VPN configuration through the AWS terraform provider. I'd like to be able to programmatically update the Unifi configuration with the pertinent info to finish standing up the VPN tunnel.

[DDxPlague]

Hoping to be able to configure these fields as well as the advanced details section.

[markti]

I would like to do the same thing on Azure ❤️