Allow custom type to be streamed and use native response

[TomeHirata]

This PR increases the extensibility of dspy.Type and allows users to define a custom type that can be streamed or use native LM response. This is useful when users want to use provider specific response field such as tool call or citation.

Following methods are introduced into dspy.Type

• is_streamable: Whether the custom type is streamable
• parse_stream_chunk: Parse a stream chunk into the custom type
• use_native_response: Whether to use the native response of the LM such as function calling

[okhat]

No concerns on a quick scan. If @chenmoneygithub approves, feel free to merge.

[chenmoneygithub]

Very clean and cool! Left some comments for discussion.

[TomeHirata]

@chenmoneygithub I've moved the native response type config to adapter, can you take a look again?