Isolate "post sticky comment" logic

[m-kuhn]

@nyalldawson requested a direct download link in a comment once we get Qt6 builds for windows ready. See #57414 (comment)

I didn't want to spam the PR's with more bot comments, so this adds a way to post multiple sections from different workflows into one sticky comment in a reusable way. I hope this will lead to more streamlined and better service comments.

It can be used by adding a step that "schedules" the comment, with a marker (e.g. mingw64) that will delimit the beginning and the end of the corresponding comment section for future replacement.

      - name: Schedule download comment
        uses: ./.github/actions/post_sticky_comment
        with:
          marker: mingw64
          body: |
            ### 🪟 Windows builds
            Download [Windows builds of this PR for testing](${{ steps.artifact-win64.outputs.artifact-url }}).
            Debug symbols for this build are available [here](${{ steps.artifact-win64-debug.outputs.artifact-url }}).";
            *(Built from commit ${{ github.event.pull_request.head.sha }})*
          pr: ${{ github.event.number }}

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit c78be25)