feat: Allow transforming sse chunk response

[grafail]

Problem
Currently it is more complicated handling sse output from custom remote inference engines that do not follow the OpenAI spec

Success Criteria
Ability to transform the response similar to how "transformResponse" is used in non streaming cases

Additional context
Perhaps a function could be passed and used around this line to customise how an sse chunk is handled

https://github.com/janhq/jan/blob/0bad1a479f642c8c5183017641656d52c4b33c61/core/src/browser/extensions/engines/helpers/sse.ts#L74

[louis-jan]

@grafail how do you see a transformResponse being embedded into a request? I see this as a client responsibility, where the SSE protocol is more or less just a communication method. We are working on client SDKs in both JS and Python, so integration could be easier in this case.

[grafail]

I was thinking about this in the same way that transformResponse is used to make other LLMs compatible when not streaming (e.g. https://github.com/janhq/jan/blob/0cae7c97612ba4f6a3387f7f72c56e065711cacf/extensions/inference-anthropic-extension/src/index.ts#L87). Effectively a way to adjust the format of my sse response to the one Jan is expecting without having to rewrite the entire requestInference logic.