chore: Add Renovate configuration

Nothing watched the pinned action versions in this repository, so the
publish action stayed on a September 2025 commit for a year and broke the
1.7.3 release. Renovate would have moved it well before then.

The config extends the shared SDK Python preset, which is the same one
python-server-sdk uses. That preset manages GitHub Actions and disables
the Python package managers, so Renovate does not touch pyproject.toml.

Author: keelerm84

.github/renovate.json

@@ -0,0 +1,4 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["github>launchdarkly/gh-actions//renovate/sdk-python"]
+}