Expose a few more class and functions to vertex (#165)

* Update vertex_hooks.dart

Expose UsageMetadata and parse functions to vertex

* Remove UsageMetadata export

This will be exported from the main library in #168.

Sort the remaining exported members.

---------

Co-authored-by: Nate Bosch <nbosch@google.com>

pkgs/google_generative_ai/lib/src/vertex_hooks.dart

@@ -17,5 +17,12 @@
 /// These APIs should not be exposed through the public libraries.
 library;
 
-export 'api.dart' show countTokensResponseFields, createCountTokensResponse;
+export 'api.dart'
+    show
+        countTokensResponseFields,
+        createCountTokensResponse,
+        parseBatchEmbedContentsResponse,
+        parseCountTokensResponse,
+        parseEmbedContentResponse,
+        parseGenerateContentResponse;
 export 'model.dart' show createModelWithBaseUri;