Add getOwn utility function

ts/util/getOwn.ts

@@ -0,0 +1,13 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { has } from 'lodash';
+
+// We want this to work with any object, so we allow `object` here.
+// eslint-disable-next-line @typescript-eslint/ban-types
+export function getOwn<TObject extends object, TKey extends keyof TObject>(
+  obj: TObject,
+  key: TKey
+): TObject[TKey] | undefined {
+  return has(obj, key) ? obj[key] : undefined;
+}