feat: Update Pull Request Template (#3641)

* Update PULL_REQUEST_TEMPLATE.md

* Updated Functional Tests to be Conditional

Co-authored-by: Mehmet Nuri Deveci <5735811+mndeveci@users.noreply.github.com>
Co-authored-by: Jacob Fuss <32497805+jfuss@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,19 +1,24 @@
 #### Which issue(s) does this change fix?
 <!-- Use the format #<issue-number>, e.g. #42 -->
 
+
 #### Why is this change necessary?
 
+
 #### How does it address the issue?
 
+
 #### What side effects does this change have?
 
-#### Checklist
+
+#### Mandatory Checklist
+**PRs will only be reviewed after checklist is complete**
 
 - [ ] Add input/output [type hints](https://docs.python.org/3/library/typing.html) to new functions/methods
-- [ ] Write design document ([Do I need to write a design document?](https://github.com/aws/aws-sam-cli/blob/develop/DEVELOPMENT_GUIDE.md#design-document))
-- [ ] Write unit tests
-- [ ] Write/update functional tests
+- [ ] Write design document if needed ([Do I need to write a design document?](https://github.com/aws/aws-sam-cli/blob/develop/DEVELOPMENT_GUIDE.md#design-document))
+- [ ] Write/update unit tests
 - [ ] Write/update integration tests
+- [ ] Write/update functional tests if needed
 - [ ] `make pr` passes
 - [ ] `make update-reproducible-reqs` if dependencies were changed
 - [ ] Write documentation