[1.17] Add Conversation API new fields

Update the Conversation (Alpha2) client to support the latest API surface so it matches the Conversation API reference.

For the request add:
- responseFormat – Optional. Structured output via a JSON Schema object
- promptCacheRetention – Optional. Prompt cache retention.

For the response add:
- model – Optional. Model identifier used for the completion.
- usage – Optional. Token usage

Reference:

- Proto/API definitions: Check the ai.proto file changes in this PR https://github.com/dapr/dapr/pull/9316/changes#diff-cc02342970a5222eef2c5aee936883611b8457feb8d3afd80763c3ebd93e3c5e
- Docs PR: https://github.com/dapr/docs/pull/5027
- Go SDK: https://github.com/dapr/go-sdk/pull/801
- Python SDK: https://github.com/dapr/python-sdk/pull/902