Support Prompt Caching for Anthropic

[MichaelDoyle]

This one is really interesting. Not sure the best way to go about it (extend Genkit's message abstraction, middleware, etc). Worth additional discussion.

See: https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching

[chrisraygill]

@MichaelDoyle is this available through Anthropic on Vertex AI? Otherwise, we can generally make this possible to implement, but the actual change will need to happen in the community plugin repo.

[MichaelDoyle]

Agreed - I suspect there is a brief discussion that needs to happen on "how" we want this to be supported in the core library, and then implementation details fall into the plugins.

When I get some free cycles I can look closer at Vertex. I suspect it might be supported in the Anthropic SDK but not Vertex AI SDK. That's how it appears to me based on the change log: anthropics/anthropic-sdk-typescript@vertex-sdk-v0.4.0...vertex-sdk-v0.4.1