Adding datastore Key.(to|from)_legacy_urlsafe.

datastore/.flake8

@@ -1,5 +1,10 @@
 [flake8]
 exclude =
+  # Datastore includes generated code in the manual layer;
+  # do not lint this.
+  google/cloud/datastore/_onestore_v3_pb2.py,
+
+  # Standard linting exemptions.
   __pycache__,
   .git,
   *.pyc,

datastore/google/cloud/datastore/_onestore_v3.proto

@@ -0,0 +1,16 @@
+syntax = "proto2";
+
+message Reference {
+  required string app = 13;
+  optional string name_space = 20;
+  required Path path = 14;
+  optional string database_id = 23;
+}
+
+message Path {
+  repeated group Element = 1 {
+    required string type = 2;
+    optional int64 id = 3;
+    optional string name = 4;
+  }
+}