feat: implement basic retry support in runtime (#487)

Author: ianbotsf

docs/design/README.md

@@ -17,3 +17,4 @@ Start here for an overview:
 * [Modeled errors](modeled-errors.md)
 * [Nullable properties in SDK domain types](nullable-properties-in-sdk-domain-types.md) (coming soon)
 * [Pagination](paginators.md)
+* [Retries](retries.md)