Added system instruction support.

[kddlb]

I'm not sure if it's up to Google's coding standards.

[devoncarew]

Interesting; I found some docs for this feature here: https://ai.google.dev/docs/system_instructions. Do you have any links to API docs for how this is exposed in other sdks (Python, REST, ...)?

[devoncarew]

cc @natebosch

[kddlb]

I looked at the Python version of the SDK. https://github.com/googleapis/google-cloud-python/blob/d9222a97786ce1badae4561410ca5e09386a3377/packages/google-ai-generativelanguage/google/ai/generativelanguage_v1beta/types/generative_service.py#L144

[natebosch]

Thanks for the PR!

There were a few changes we would have needed to make here, but I had a local branch with those changes almost all ready so I went ahead and pushed up that PR to shorten the cycle time. Primarily my version accepts Content? instead of String? to match the backend API a little more directly. In the Python SDK I think they tend to be a little looser with types and accept multiple formats. If we had union types or method overloads I'd probably do the same and accept either String or Content.

I'll close this PR for now and run with #119