ci: first part of the source-git automation - commit linter

Add a GitHub Workflow that is triggered on `workflow_run` events.
It uses metadata provided by `redhat-plumbers-in-action/gather-pull-request-metadata`
GitHub Action to get the PR number and the commit metadata.
The commit metadata is then used to check if the commit message contains
all required information (tracker and upstream reference). GitHub Action
responsible for commit verification `redhat-plumbers-in-action/advanced-commit-linter`
is configured via the `advanced-commit-linter.yml` file.

rhel-only

Related: #2190153

.github/advanced-commit-linter.yml

@@ -0,0 +1,23 @@
+policy:
+  cherry-pick:
+    upstream:
+      - github: systemd/systemd
+      - github: systemd/systemd-stable
+    exception:
+      note:
+        - rhel-only
+  tracker:
+    - keyword:
+        - 'Resolves: #?'
+        - 'Related: #?'
+        - 'Reverts: #?'
+      issue-format:
+        - '\d+$'
+      url: 'https://bugzilla.redhat.com/show_bug.cgi?id='
+    - keyword:
+        - 'Resolves: '
+        - 'Related: '
+        - 'Reverts: '
+      issue-format:
+        - 'RHEL-\d+$'
+      url: 'https://issues.redhat.com/browse/'

.github/workflows/source-git-automation.yml

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+name: Source git Automation
+on:
+  workflow_run:
+    workflows: [ Gather Pull Request Metadata ]
+    types:
+      - completed
+
+permissions:
+  contents: read
+
+jobs:
+  download-metadata:
+    if: >
+      github.event.workflow_run.event == 'pull_request' &&
+      github.event.workflow_run.conclusion == 'success'
+    runs-on: ubuntu-latest
+
+    outputs:
+      pr-metadata: ${{ steps.Artifact.outputs.pr-metadata-json }}
+
+    steps:
+      - id: Artifact
+        name: Download Artifact
+        uses: redhat-plumbers-in-action/download-artifact@v1
+        with:
+          name: pr-metadata
+
+  commit-linter:
+    needs: [ download-metadata ]
+    runs-on: ubuntu-latest
+
+    outputs:
+      validated-pr-metadata: ${{ steps.commit-linter.outputs.validated-pr-metadata }}
+
+    permissions:
+      statuses: write
+      pull-requests: write
+
+    steps:
+      - id: commit-linter
+        name: Lint Commits
+        uses: redhat-plumbers-in-action/advanced-commit-linter@v1
+        with:
+          pr-metadata: ${{ needs.download-metadata.outputs.pr-metadata }}
+          token: ${{ secrets.GITHUB_TOKEN }}