[REFACTOR] builder pattern for complex types

[cachebag]

Creating VpnCredentials and EapOptions requires setting all fields manually, which is error-prone.

We could do something like this:

let creds = VpnCredentials::builder()
    .name("MyVPN")
    .wireguard()
    .gateway("vpn.example.com:51820")
    .private_key("...")
    .address("10.0.0.2/24")
    .add_peer(peer)
    .build()?;