Add Dependabot config for GitHub Actions

We have some 3rd party GitHub Action's we're pinning for determinism
and security, however we'd like these to stay up to date.

Initial reviewers are a mix of security and internal tools folks, chosen
to be able to perform initial triage.

.github/dependabot.yml

@@ -169,3 +169,16 @@ updates:
       - jentfoo
       - rosstimothy
       - zmb3
+
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: weekly
+      day: monday
+      time: "09:00"
+      timezone: "America/Los_Angeles"
+    reviewers:
+      - wadells
+      - jentfoo
+      - fheinecke
+      - camscale