Expand explicit context caching to agent tools and sub agents

[danhnguyen-ct]

The new feature explicit context caching now only works on root agent while many LLM requests are happened within agent tools or sub agents.

Please expand the feature to also support agent tools or sub agents that can help to save cost.

[klateefa]

@danhnguyen-ct Thank you for reporting this. I understand the request is currently under review. Please keep me posted if anything else is needed. Thanks for your patience.