[feat] gemini context caching

[sxlijin]

From codager on discord:

**[9:22 PM] OP codager:

I have 2 use cases

1. I want to cache the system prompt across multiple invocations
2. I want to upload the file to gemini and use that across prompts to get token benefits.

Are these possible through BAML?

[linear]

BAML-530