Add ContentHandler templates for common models

[cj-zhang]

Introduce some ContentHandler templates for common models i.e. the Llama family. Could save users time and also provide an example of how to write their own content handlers.

class LlamaContentHandler(LLMContentHandler):
    content_type: str = "application/json"
    accepts: str = "application/json"

    def transform_input(self, inputs: str, model_kwargs) -> bytes:
        input_str = json.dumps({"inputs": inputs, **model_kwargs})
        return input_str.encode('utf-8')

    def transform_output(self, output):
        response_json = json.loads(output.read().decode("utf-8"))
        return response_json["generated_text"]