[flutter release] Add cherry pick template for pull request description (#139590)

context:  https://github.com/flutter/flutter/pull/139524#discussion_r1416052456

Adds a pull request template for automated flutter cherry picks.
This template is adapted from  [web form cherry pick issue template](https://cs.opensource.google/flutter/flutter/+/master:.github/ISSUE_TEMPLATE/7_cherry_pick.yml) since PR template doesn't support web form.

This would provide the template to allow https://github.com/flutter/flutter/pull/139524 to land

.github/PR_TEMPLATE/PULL_REQUEST_CP_TEMPLATE.md

+**This pull request is opened against a release branch.<br>
+To request a cherry pick of a commit, please fill in the form below.** (Questions with an asterisk are required.)<br>
+
+**To fill in the form, you can edit this PR description and type your answers after the 'My Answer' keywords. <br>
+A flutter domain expert will evaluate this cherry pick request shortly after ALL questions are answered.**
+
+* Issue Link: What is the link to the issue this cherry-pick is addressing?<br>
+
+<pre>
+  <b>My Answer:</b>
+
+
+</pre>
+
+* Changelog Description: Explain this cherry pick in one line that is accessible to most Flutter developers
+        See https://github.com/flutter/flutter/wiki/Hotfix-Documentation-Best-Practices for examples (Bug fix, feature, docs update, ...)<br>
+
+<pre>
+  <b>My Answer:</b>
+
+
+</pre>
+
+* Impacted Users: Approximately who will hit this issue (ex. all Flutter devs, Windows developers, all end-customers, apps using X framework feature)?<br>
+
+<pre>
+  <b>My Answer:</b>
+
+
+</pre>
+
+* impact_description: What is the impact (ex. visual jank on Samsung phones, app crash, cannot ship an iOS app)? Does it impact development (ex. flutter doctor crashes when Android Studio is installed), or the shipping production app (the app crashes on launch)<br>
+
+<pre>
+  <b>My Answer:</b>
+
+
+</pre>
+
+* Workaround: Is there a workaround for this issue?<br>
+
+<pre>
+  <b>My Answer:</b>
+
+
+</pre>
+
+* Risk: What is the risk level of this cherry-pick?<br>
+
+<pre>
+  <b>My Answer:</b>
+  - [ ] Low
+  - [ ] Medium
+  - [ ] High
+
+</pre>
+
+* Test Coverage: Are you confident that your fix is well-tested by automated tests?<br>
+
+<pre>
+  <b>My Answer:</b>
+  - [ ] Yes
+  - [ ] No
+
+</pre>
+
+* Validation Steps: What are the steps to validate that this fix works?<br>
+
+<pre>
+  <b>My Answer:</b>
+
+
+</pre>