Add 'json_schema' alias to GrammarType.Json

[aW3st]

Adds a serde alias to GrammarType.Json such that {"type": "json_schema", "value": json_schema} deserializes correctly.

This brings the OpenAI-compatible API closer to OpenAI's official spec and makes the server more interoperable with other OpenAI-compatible servers.

Eventually it would be nice to be able to use beta.chat.completions.parse method on OpenAI's python client.

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Who can review?

@OlivierDehaene OR @Narsil

[Narsil]

Hi @aW3st,

Thanks a lot for the PR. We're unlikely to adhere to anything beta from OpenAI, or even everything that OpenAI supports.
The OpenAI adaptation layer exists for simple, convenient ubiquitous tool support from client libraries (most of which have a OpenAI compatibility layer with various degrees of support).

We intend to support those very common use cases and be as easy as possible to be consumed. However, the intent of TGI is to support the many various different open source models, which sometimes operate very differently than OpenAI's. Therefore adhering to exact OpenAI spec can become quite difficult, especially for the most advanced use cases.

That being said json_schema should probably be supported. However I don't think json_schema is equivalent to the current json, we should probably upgrade outlines which now has support for both.

Pinging @drbh in case I said anything wrong here.