chat: add FunctionGemma tool call support
Implements tool call handling for FunctionGemma models which use a
unique format: <start_function_call>call:name{key:<escape>val<escape>}<end_function_call>
Changes:
- Add COMMON_CHAT_FORMAT_FUNCTION_GEMMA enum
- Implement grammar builder for FunctionGemma format
- Implement parser for FunctionGemma tool calls
- Add template detection for <start_function_call> markers
- Add google-functiongemma.jinja template
- Add unit tests and e2e test entries
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Co-Authored-By: Claude <noreply@anthropic.com>
