Missing Caching Support

**Describe the solution you'd like**
Missing caching support equivalent to the [Python SDK](https://cloud.google.com/vertex-ai/generative-ai/docs/context-cache/context-cache-use#use-context-cache-sample-python) or [Gemini TS SDK](https://github.com/google-gemini/generative-ai-js/blob/main/samples/cache.js)

**Describe alternatives you've considered**
I can create & manage caches with raw requests to the API endpoint, but I cannot use them as the `cached_content` cannot be passed through the library to the request