fix(types): extends check of generic in WithSdkStreamMixin (#4119)

* fix(types): Fix issue with WithSdkStreamMixin

Fix an issue where WithSdkStreamMixin would apply to more properties than intended due to Distributive Conditional Types.

* fix(streams): apply mixin only to key match instead of type match

* fix(streams): update docs to indicate single key match instead of type match

Co-authored-by: George Fu <kuhe@users.noreply.github.com>

packages/types/src/serde.ts

@@ -87,12 +87,12 @@ export interface SdkStreamMixin {
 export type SdkStream<BaseStream> = BaseStream & SdkStreamMixin;
 
 /**
- * Indicates that any members of type T
- * that were of type T[StreamKey] have been extended
+ * Indicates that the member of type T with 
+ * key StreamKey have been extended
  * with the SdkStreamMixin helper methods.
  */
 export type WithSdkStreamMixin<T, StreamKey extends keyof T> = {
-  [key in keyof T]: T[key] extends T[StreamKey] ? SdkStream<T[StreamKey]> : T[key];
+  [key in keyof T]: key extends StreamKey ? SdkStream<T[StreamKey]> : T[key];
 };
 
 /**