[FR]: Add support for json_schema in the playground

[jverre]

Proposal summary

Today the playground supports many parameters but not json_schema, would be good to have this supported as well

Motivation

JSON schema is very commonly used to ensure that the output follows a specific json_schema (it's actually used as part of the LLM as a Judge metrics in Opik).

[dsblank]

Thank you @jverre! We have added this feature request to the roadmap as item OPIK-837