action: validate changelog fragment (#1488)

(cherry picked from commit 259682d)

.github/workflows/changelog.yml

@@ -0,0 +1,17 @@
+name: Changelog
+on: [pull_request]
+
+jobs:
+  fragments:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - name: Fetch Go version from .go-version
+      run: echo "GO_VERSION=$(cat .go-version)" >> $GITHUB_ENV
+    - uses: actions/setup-go@v3
+      with:
+        go-version: ${{ env.GO_VERSION }}
+    - name: check pr-has-fragment
+      run: |
+        GOBIN=$PWD/bin go install github.com/elastic/elastic-agent-changelog-tool@latest
+        ./bin/elastic-agent-changelog-tool pr-has-fragment --repo ${{ github.event.repository.name }} ${{github.event.number}}