Create aws/SDK_CHANGELOG.md to track SDK changes during development (smithy-lang#692)

* Backfill AWS SDK changelog

* Add reminder to update SDK changelog to PR template

* Update SDK changelog for latest release

Author: jdisanti

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,6 +11,7 @@
 <!--- see how your change affects other areas of the code, etc. -->
 
 ## Checklist
-- [ ] I have updated the CHANGELOG
+- [ ] I have updated `CHANGELOG.md`
+- [ ] I have updated `aws/SDK_CHANGELOG.md` if applicable
 
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