Add Function Calling Example

[h0rv]

No description provided.

[h0rv]

When running this example, I am seeing the following error:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: APIError { message: "400: {\n  \"error\": {\n    \"message\": \"Invalid schema for function 'Searches': None is not valid under any of the given schemas.\",\n    \"type\": \"invalid_request_error\",\n    \"param\": \"tools[0].function.parameters\",\n    \"code\": \"invalid_function_parameters\"\n  }\n}" }', /home/horv/.cargo/registry/src/github.com-1ecc6299db9ec823/instructor-ai-0.1.0/src/lib.rs:117:55
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[h0rv]

Enums are not yet supported - will work on getting them working before merging

[jxnl]

@ivanleomk got some ideas on enums