Add a functionCalls getter (#118)

This is similar to the `text` getter, except that it does not check for
blocked prompt or blocked response outputs. Instead returns an empty
list if there are no functions for any reason. This matches the behavior
of the SDKs implemented in other languages.

pkgs/google_generative_ai/lib/src/api.dart

@@ -79,6 +79,15 @@ final class GenerateContentResponse {
       [Candidate(), ...] => null,
     };
   }
+
+  /// The function call parts of the first candidate in [candidates], if any.
+  ///
+  /// Returns an empty list if there are no candidates, or if the first
+  /// candidate has no [FunctionCall] parts. There is no error thrown if the
+  /// prompt or response were blocked.
+  Iterable<FunctionCall> get functionCalls =>
+      candidates.firstOrNull?.content.parts.whereType<FunctionCall>() ??
+      const [];
 }
 
 final class EmbedContentResponse {

samples/dart/bin/function_calling.dart

@@ -45,8 +45,7 @@ void main() async {
   final content = [Content.text(prompt)];
   final response = await model.generateContent(content);
 
-  final functionCalls =
-      response.candidates.first.content.parts.whereType<FunctionCall>();
+  final functionCalls = response.functionCalls.toList();
   if (functionCalls.isEmpty) {
     print('No function calls.');
     print(response.text);