Add PR template for soda-extensions reminder (#2494)

We've had a few times where the soda-extensions package was broken as a
result of changes in soda-core. There is a workflow in place to validate
they work together correctly, but this is a manual process.

This PR adds a template to promote good PR practices, including a
reminder to run the soda-extensions workflow for validating
compatibility.

Author: mivds

.github/pull_request_template.md

@@ -0,0 +1,15 @@
+## Description
+
+Please provide a description of your changes:
+
+- What problem are you solving?
+- Any expected impact on downstream packages/services?
+- Reference issue # if available.
+
+
+## Checklist
+
+- [ ] I added a test to verify the new functionality.
+- [ ] I verified this PR does not break [soda-extensions][1].
+
+[1]: (https://github.com/sodadata/soda-extensions/actions/workflows/main.workflow.yaml)