refactor ChatCompletionsRequest

[declark1]

NOTE: I want to test this a bit more and add a deserialization unit test, but opening this for review now. This seems to be the only approach that I can see working for this.

Changes:

• Refactors ChatCompletionRequest to a subset of fields used by the orchestrator (detectors, stream, model, messages) and inner: Map<String, Value> containing the full request
  • Uses #[serde(try_from = "Map<String, Value>")] for deserialization, with the validation and extraction of the fields noted above happening in the TryFrom<Map<String, Value>> for ChatCompletionsRequest implementation
  • Has custom Serialize implementation, which just serializes the inner request
• Updated chat completions detection integration tests to use the json!() macro instead of ChatCompletionsRequest as the latter now needs to be built via deserialization rather than directly
• Dropped orchestrator_validation_error integration test scenario for unknown fields and these can no longer be validated. We only validate fields used by the service, with full validation now delegated to the downstream server.

Closes #367

[mdevino]

Left a few nits. Looks like there are a couple of conflicts with the current code base due to recent merges.

[declark1]

Added unit test, addressed initial review comments, rebased to bring in new tests, and updated the new validation test scenarios accordingly to use json!(). Now ready for final review.

[mdevino]

LGTM! Left a few nit comments.

[evaline-ju]

LGTM!