Add function calling support (google-gemini#116)

Sources/GoogleAI/Chat.swift

@@ -175,7 +175,7 @@ public class Chat {
         case let .text(str):
           combinedText += str
 
-        case .data(mimetype: _, _):
+        case .data, .functionCall, .functionResponse:
           // Don't combine it, just add to the content. If there's any text pending, add that as
           // a part.
           if !combinedText.isEmpty {