treewide: add license header and enable haslicense linter

Change-Id: I873a8d4082d75e8f813d8a726a41187eea7a065e
Reviewed-on: https://review.monogon.dev/c/monogon/+/3825
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/cloud/shepherd/manager/manager.go b/cloud/shepherd/manager/manager.go
index 3ae7854..ae8115e 100644
--- a/cloud/shepherd/manager/manager.go
+++ b/cloud/shepherd/manager/manager.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 // Package manager, itself a part of BMaaS project, provides implementation
 // governing Equinix bare metal server lifecycle according to conditions set by
 // Bare Metal Database (BMDB).