Allow use of ppkarwasz/fetch-metadata alongside dependabot/fetch-metadata

This PR allows the usage of the `ppkarwasz/fetch-metadata` GitHub Action as an alternative to `dependabot/fetch-metadata` in ASF repositories.

The `ppkarwasz/fetch-metadata` action is a personal improvement of the original `dependabot/fetch-metadata`, adding support for grouped Dependabot pull requests, a feature that is currently missing from the upstream action. The implementation has already been reviewed and approved by the Dependabot team (see dependabot/fetch-metadata#632), but the upstream project has been inactive for several months, likely due to reduced maintenance capacity at GitHub. This has prevented the improvement from being merged and released.

### Why this change is needed

In Apache Logging Services, every pull request must include a changelog entry. Previously, under CTR, we used a workflow that automatically added the changelog entry and merged the PR.

Since switching to RTC, this automation can no longer complete the merge step, resulting in repositories accumulating unmerged Dependabot PRs that must be:

* manually reviewed,
* updated with an empty commit to re-trigger required status checks,
* and merged by hand.

We already have an improved workflow in place (see apache/logging-parent#419) that provides:

* **Security enhancements** through separation of privileged and unprivileged workflows
  (`ppkarwasz/fetch-metadata` is used only in the unprivileged workflow),
* **Automatic merge using `auto-merge` instead of manual merging**, and
* **Support for grouped Dependabot PRs** (reducing noise to ~1 PR per repository per month).

The final item, grouped PR support, requires the `ppkarwasz/fetch-metadata` action.

Author: ppkarwasz

actions.yml

@@ -623,6 +623,10 @@ potiuk/cancel-workflow-runs:
   '*':
     expires_at: 2025-08-01
     keep: true
+ppkarwasz/fetch-metadata:
+  14da1d746fb4c6f05dbc353e4fd619a1065c8ff6:
+    expires_at: 2025-10-01
+    tag: v0.1.0
 pre-commit/action:
   2c7b3805fd2a0fd8c1884dcaebf91fc102a13ecd:
     expires_at: 2050-01-01