Add JSON generation method

[rlouf]

In this PR we add a new generation method that can generate valid JSON following a Pydantic mode or JSON Schema specification.

We first parse the (possibly nested) JSON schema into a generation schedule, which is a list of strings that represent the JSON structure, and annotations that correspond to the field specifications. The following annotations are currently supported, with a minimal set of functionalities:

• string
• int
• float
• bool
• Union[type1, type2]
• enum
• List[type]
• List[Union[type1, type2]]
• Optional[type]
• Nested schema
• maxLength for strings
• minLength for strings

The tests demonstrate how to build a schedule from Pydantic models, using the models' schema_json method.

We also add an outlines.text.generate.json function that parses a JSON schema, turns it into a regex and instantiates the Regex class.

[havietisov]

off topic : would be interesting to see constrained generation of HTML

[rlouf]

off topic : would be interesting to see constrained generation of HTML

Coming with #178