Update helm chart from ci (#1167)

# What this PR does

Add github action workflow to automatically bump oncall helm chart version on each release and create a PR with this change

## Which issue(s) this PR fixes

## Checklist

- [ ] Tests updated
- [ ] Documentation added
- [ ] `CHANGELOG.md` updated

.github/workflows/helm_release_pr.yml

@@ -0,0 +1,45 @@
+name: Create PR to release updated oncall Helm chart
+
+on:
+  push:
+    tags:
+      # https://semver.org/#is-there-a-suggested-regular-expression-regex-to-check-a-semver-string
+      # with added v in front of semver2 regex
+      - "^v(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+([0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$"
+
+jobs:
+  update-helm-chart-versions:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Define app_version and helm version
+        id: tags
+        run: |
+          # Strip git ref prefix from version
+          APP_VERSION=$(echo "${{ github.ref }}" | sed -e 's,.*/\(.*\),\1,')
+          # Strip "v" prefix from tag name
+          [[ "${{ github.ref }}" == "refs/tags/"* ]] && HELM_VERSION=$(echo $APP_VERSION | sed -e 's/^v//')
+          echo "::set-output name=app_version::$APP_VERSION"
+          echo "::set-output name=helm_version::$HELM_VERSION"
+
+      - name: Update oncall Helm chart Chart.yaml
+        uses: fjogeleit/yaml-update-action@v0.12.3
+        with:
+          valueFile: 'helm/oncall/Chart.yaml'
+          branch: helm-release/${{ steps.tags.outputs.helm_version }}
+          targetBranch: main
+          masterBranchName: main
+          createPR: 'true'
+          description: "Merge this PR to `main` branch to start another 
+            [github actions job](https://github.com/grafana/oncall/blob/dev/.github/workflows/helm_release.yml) 
+            that will release the updated version of the chart 
+            (version: ${{ steps.tags.outputs.helm_version }}, appVersion: ${{ steps.tags.outputs.app_version }}) 
+            into `grafana/helm-charts` helm repository. \n\n
+            This PR was created automatically by this 
+            [github action](https://github.com/grafana/oncall/blob/dev/.github/workflows/helm_release_pr.yml)."
+          message: 'Release oncall Helm chart ${{ steps.tags.outputs.helm_version }}'
+          changes: |
+            {
+              "version": "${{ steps.tags.outputs.helm_version }}",
+              "appVersion": "${{ steps.tags.outputs.app_version }}"
+            }