Update encodable type class to have optional prompt prefix (closes #498)

[kiranandcode]

• Updates EncodableAs to have an optional class method encoding_instructions

class EncodableAs[T, U](ABC):
    ...
    @classmethod
    def encoding_instructions(cls) -> str | None:
        """Optional instructions to be prefixed onto synthesis prompts to tune the encoding of the result."""
        return None

• Updates the default implementation for format_model_input to retrieve the encodable type for the return type and append a prompt prefix if encoding instructions is defined.

    ret_type = template.__signature__.return_annotation
    origin = typing.get_origin(ret_type)
    ret_type = ret_type if origin is None else origin
    ret_type_encoder = type_to_encodable_type(ret_type)
    prompt_prefix = ret_type_encoder.encoding_instructions()

    if prompt_prefix:
        prefix: list[ChatCompletionTextObject] = [
            {"type": "text", "text": prompt_prefix}
        ]
        prompt = prefix + prompt

Closes #498