NetBird SSH (#361)

This PR adds support for SSH access through the NetBird network
without managing SSH skeys.
NetBird client app has an embedded SSH server (Linux/Mac only) 
and a netbird ssh command.

util/membership_windows.go

@@ -0,0 +1,12 @@
+package util
+
+import "golang.zx2c4.com/wireguard/windows/elevate"
+
+// IsAdmin returns true if user has admin privileges
+func IsAdmin() bool {
+	adminDesktop, err := elevate.IsAdminDesktop()
+	if err == nil && adminDesktop {
+		return true
+	}
+	return false
+}