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.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@OlivierDehaene OR @Narsil