[actions] Add GitHub action to compute the changelog for maestro bumps. (#14984)

And add that changelog as a comment to the PR. If there already is a comment
with a changelog, then update that comment instead.

.github/workflows/maestro-changelog.yml

@@ -0,0 +1,59 @@
+name: Add changelog for Maestro bump
+on: pull_request
+
+jobs:
+  add-changelog:
+    name: Add changelog
+    runs-on: ubuntu-latest
+    if: github.actor == 'dotnet-maestro[bot]'
+
+    steps:
+    - name: 'Compute changelog'
+      run: |
+        set -exo pipefail
+        git clone https://github.com/spouliot/dotnet-tools
+        cd dotnet-tools/changelog
+        dotnet run https://github.com/$GITHUB_REPOSITORY/pull/${GITHUB_REF_NAME/\/*/} > changelog.txt 2>&1
+
+        # need to replace newlines with actual "\n"
+        rm -f changelog2.txt
+        echo "CHANGELOG_MESSAGE<<EOF" >> changelog2.txt
+        cat changelog.txt | sed -e ':a' -e 'N' -e '$!ba' -e 's/\n/\\n/g' >> changelog2.txt
+        echo EOF >> changelog2.txt
+        cat changelog2.txt
+
+        # export the changelog message
+        cat changelog2.txt >> $GITHUB_ENV
+
+    - name: 'Add changelog'
+      uses: actions/github-script@v6
+      with:
+        github-token: ${{secrets.GITHUB_TOKEN}}
+        script: |
+          // check if we've already added a changelog to this PR, and if so, update that comment, otherwise add a new comment
+          var commentId = ""
+          await github.paginate (github.rest.issues.listComments,
+          {
+            owner: context.repo.owner,
+            repo: context.repo.repo,
+            issue_number: context.issue.number
+          }).then ((comments) =>
+          {
+            const changelogComment = comments.find(comment => comment.body.includes (".net ChangeLog for") && comment.user.login == 'github-actions[bot]')
+            commentId = changelogComment.id
+          })
+          if (commentId == "") {
+            github.rest.issues.createComment({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: context.issue.number,
+              body: '${{ env.CHANGELOG_MESSAGE }}'
+            })
+          } else {
+            github.rest.issues.updateComment({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              comment_id: commentId,
+              body: '${{ env.CHANGELOG_MESSAGE }}'
+            })
+          }