Explicitly add the do-not-merge/hold label when cherry-pick plugin created a PR with soft conflicts

[wuhuizuo]

Summary

When a cherry-pick PR is created with soft conflicts, explicitly add blocking label to prevent the merging and give comment feedback.

Implementation

It is implemented in ti-community-infra/tichi#1192

Effects

1. cherry-pick plugin created a PR with soft conflicts.
   
2. Give feedback with comments in the main PR.
   
3. Add a blocking label to the PR created by cherry-pick plugin and feedback with comments.
   
   

[wuhuizuo]

Thanks for @BornChanger's suggestion.