.github/dependabot.yml: Enable dependabot (#169)

## Description

Enables dependabot in this repo.

This GitHub action will automatically create pull requests and
summarize the dependency details. Because it is a pull request,
the CI system will validate the dependency update in the pull
request.

Configures dependabot for:

1. PIP module updates
2. Submodule updates
3. GitHub action updates

- [ ] Breaking change?
- Will this change break pre-existing builds or functionality without
action being taken?
  **No**

## How This Was Tested

Verified config in other repos.

## Integration Instructions

N/A - Impacts this repo

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

.github/dependabot.yml

@@ -0,0 +1,34 @@
+## @file
+# Dependabot configuration file to enable GitHub services for managing and updating
+# dependencies.
+#
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+# Please see the documentation for all configuration options:
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+##
+
+version: 2
+updates:
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    commit-message:
+      prefix: "pip"
+
+  - package-ecosystem: "gitsubmodule"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    commit-message:
+      prefix: "submodule"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    commit-message:
+      prefix: "GitHub Action"