Support json schema for response format type in OpenAIChatCompletionClient

[ekzhu]

Resolves #5982

This PR adds support for json_schema as a response_format type in OpenAIChatCompletionClient. This is necessary because it allows the client to be serialized along with the schema. If user use response_format=SomeBaseModel, the client cannot be serialized.

Usage:

# Structured output response, with a pre-defined JSON schema.

OpenAIChatCompletionClient(...,
response_format = {
    "type": "json_schema",
    "json_schema": {
        "name": "name of the schema, must be an identifier.",
        "description": "description for the model.",
        # You can convert a Pydantic (v2) model to JSON schema
        # using the `model_json_schema()` method.
        "schema": "<the JSON schema itself>",
        # Whether to enable strict schema adherence when
        # generating the output. If set to true, the model will
        # always follow the exact schema defined in the
        # `schema` field. Only a subset of JSON Schema is
        # supported when `strict` is `true`.
        # To learn more, read
        # https://platform.openai.com/docs/guides/structured-outputs.
        "strict": False,  # or True
    },
},
)

[jorge-wonolo]

Works perfect to solve the issue filed under #5982

[lspinheiro]

I think there are still some issues with the tests, but the main change looks good.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.70%. Comparing base (09d8d34) to head (9616b5c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5988      +/-   ##
==========================================
+ Coverage   76.68%   76.70%   +0.02%     
==========================================
  Files         191      191              
  Lines       13152    13164      +12     
==========================================
+ Hits        10085    10097      +12     
  Misses       3067     3067              

Flag	Coverage Δ
unittests	76.70% <100.00%> (+0.02%)	⬆️

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