aws: Remove plugin credential providers (#391)

Removing plugin credential provider from the v2 SDK developer preview. This feature may be made available as a separate module.

Author: skotambkar

CHANGELOG_PENDING.md

@@ -1,6 +1,11 @@
 Services
 ---
 
+Deprecations
+---
+* `aws`: Removes plugin credential provider ([#391](https://github.com/aws/aws-sdk-go-v2/pull/391))
+  * Removing plugin credential provider from the v2 SDK developer preview. This feature may be made available as a separate module.
+  
 SDK Features
 ---