Adding support for tool calling into the providers

[csells]

This change allows the user to handle tool calls during the processing of LLM requests, e.g.

class ChatPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) => Scaffold(
    appBar: AppBar(title: const Text(App.title)),
    body: LlmChatView(
      provider: GeminiProvider(
        model: GenerativeModel(
          model: 'gemini-2.0-flash',
          apiKey: geminiApiKey,
          tools: [
            Tool(
              functionDeclarations: [
                FunctionDeclaration(
                  'get_temperature',
                  'Get the current local temperature',
                  Schema.object(properties: {}),
                ),
                FunctionDeclaration(
                  'get_time',
                  'Get the current local time',
                  Schema.object(properties: {}),
                ),
              ],
            ),
          ],
        ),
        onFunctionCall: _onFunctionCall,
      ),
    ),
  );

  Future<Map<String, Object?>?> _onFunctionCall(
    FunctionCall functionCall,
  ) async {
    // note: just as an example, we're not actually calling any external APIs
    return switch (functionCall.name) {
      'get_temperature' => {'temperature': 60, 'unit': 'F'},
      'get_time' => {'time': DateTime(1970, 1, 1).toIso8601String()},
      _ => throw Exception('Unknown function call: ${functionCall.name}'),
    };
  }
}

issues that are fixed by this PR

• How Can I get functionCalls? #98

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.