Returning an object instead of plain text for generation

[RobinPicard]

Currently, Outlines's generation functions return a plain string or a list of strings in the case of batching. This is convenient for end users as they can directly use the output without accessing a specific field, but it's quite constraining for us. There are features for which the possibility to return different things encapsulated in an object would be very useful. I have in mind in particular:

• Reasoning models: we would like to separate the final answer from the reasoning of the model. Otherwise, the end user would have to do the parsing themselves, which is pretty inconvenient
• Tools: we would like to have a way of distinguishing different types of messages as there would be tool calls and tool responses instead of regular text. Additionally, some models require tool calls to include an id, so we would probably need to return a dictionary in any case

Ideally, the object we would return could be used as a message in the Chat input type.