Add Dependabot configuration for automated dependency updates (#2230)

Add dependabot.yml to enable automated dependency updates for:
- GitHub Actions workflows (weekly)
- Python pip packages (weekly)

This follows OpenSSF Best Practices Badge recommendations and
is part of the Step Security hardening measures.

Part of #2035

Signed-off-by: pmady <pavan4devops@gmail.com>
Co-authored-by: Doug Walker <doug.walker@autodesk.com>

.github/dependabot.yml

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: BSD-3-Clause
+# Copyright Contributors to the OpenColorIO Project.
+
+version: 2
+
+updates:
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"