Add GitHub Dependabot

It will create PRs for Maven and GitHub action updates. Of course, we
cannot simply merge the PRs, because Dependabot has not singed an
Eclipse CLA, but at least we know about updates and how they affect the
build result. Then we can decide to clone the changes and commit them
manually.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: "maven"
+    directory: "/"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"