[Chore] Add AI usage confirmation to the PR template (#17977)

Author: ruanwenjun

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,8 @@
 <!--Thanks very much for contributing to Apache DolphinScheduler, we are happy that you want to help us improve DolphinScheduler! -->
 
+## Was this PR generated or assisted by AI?
+<!--(Please answer YES or NO. If YES, please specify which parts were generated or assisted by AI)-->
+
 ## Purpose of the pull request
 
 <!--(For example: This pull request adds checkstyle plugin).-->