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_unix.go

@@ -0,0 +1,12 @@
+//go:build linux || darwin
+
+package util
+
+import (
+	"os"
+)
+
+// IsAdmin returns true if user is root
+func IsAdmin() bool {
+	return os.Geteuid() == 0
+}