Add copy-pasteable cherry-picker command if it fails to cherry-pick (#…

…44316)

When cherry-picker fails to backport a PR it adds a message to
the PR explaining the failure and linking to the commit. This PR
also adds a copy&pasteable cherry-picker command that the maintainer
should run to retry it locally. It also explains what
maintainer should do after solving the conflicts.

dev/backport/update_backport_status.py

@@ -52,7 +52,23 @@ def get_failure_comment(branch: str, commit_sha_url: str, commit_sha: str):
                 <td>{branch}</td>
                 <td><a href="{commit_sha_url}"><img src='{commit_shield_url}' alt='Commit Link'></a></td>
             </tr>
-        </table>"""
+        </table>
+
+        You can attempt to backport this manually by running:
+
+        ```bash
+        cherry_picker {commit_sha[:7]} {branch}
+        ```
+
+        This should apply the commit to the {branch} branch and leave the commit in conflict state marking
+        the files that need manual conflict resolution.
+
+        After you have resolved the conflicts, you can continue the backport process by running:
+
+        ```bash
+        cherry_picker --continue
+        ```
+"""
     return comment