Allow output guardrails on stream responses

[cescoffier]

The idea is to add the possibility to define a strategy:

• full-response (default, defeat the purpose of streaming, so should be logged at build time)
• delimiter - validate the response chunk by chunk where each chunk is the accumulation of token till a defined delimiter
• size - validate the response chunk by chunk where each chunk is the accumulation of token till a defined size
• token - validate each token individually - this strategy is just there to be complete, because, I doubt it will be very useful

Be aware that the guardrail method will be called on the event loop, and for now, cannot block.