Support for functions

[phayes]

This PR implements preliminary support for functions.

Not ready for merge yet, but the skeleton is here and ready for feedback.

Everything is fairy straightforward with the exception of JSONSchema. Instead of adding a dependency on the schemars crate and requiring a fully-typed SchemaObject, which is hellish, I simply used serde_json::Value and left it up to the user to either use schemars, or construct something by hand, or deserialize something etc.

I'll add some good documentation to this before this PR is ready to merge.

[Kodylow]

Is this ready for testing/review @phayes ? Got a couple function-y things I'm trying to do haha

[phayes]

Hi @Kodylow ,

Yes! Although I snuck a bunch of other changes in here as well. Let me know if you'd like me to split it into a bunch of different PRs.

The only thing missing is documentation.

[phayes]

I've marked this ready for review:

1. Function calling works with both standard API and streaming API
2. To aid in debugging function calls (which is often needed), I've added an optional dependency on the "log" crate and added some debugging log points.
3. I've documented calling functions in lib.rs for both the regular API and the streaming API.
4. I've added better handing of stream errors (needed to have visibility into errors in calling function when streaming)

[phayes]

My apologies, as I did some final testing I ran into a bunch of small issues that are now fixed. I've also added calling_function and calling_function_stream to the examples directory. I've also impl'ed Default for ChatMessage

I don't intend on making any more changes now. Please review!

[Kodylow]

🥳