Split etcdctl into etcdctl (public API access) & etcdutl (direct surgery on files)

Motivation is as follows:

  - etcdctl we only depend on clientv3 APIs, no dependencies of bolt, backend, mvcc, file-layout
  - etcdctl can be officially supported across wide range of versions, while etcdutl is pretty specific to file format at particular version.
it's step towards desired modules layout, documented in: https://etcd.io/docs/next/dev-internal/modules/

bill-of-materials.json

@@ -485,6 +485,15 @@
 			}
 		]
 	},
+	{
+		"project": "go.etcd.io/etcd/etcdutl/v3",
+		"licenses": [
+			{
+				"type": "Apache License 2.0",
+				"confidence": 1
+			}
+		]
+	},
 	{
 		"project": "go.etcd.io/etcd/pkg/v3",
 		"licenses": [