Add changelog

Author: landonxjames

.changelog/imds-retries.md

@@ -0,0 +1,10 @@
+---
+applies_to: ["client"]
+authors: ["landonxjames"]
+references: ["aws-sdk-rust#1233"]
+breaking: false
+new_feature: true
+bug_fix: false
+---
+
+Allow IMDS clients to be configured with a user-provided `SharedRetryClassifier`.