Try function calling for tool invocation

[oliverchang]

Instead of manually encoding it ourselves inside our prompts.

https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling

When @DonggeLiu first tried this a few months ago, he ran into some issues (@DonggeLiu do you remember what they were?)

We should try this again.

[oliverchang]

@DavidKorczynski can you help us take a look at this?

[DonggeLiu]

@DonggeLiu do you remember what they were?

Documented here:
#731 (comment)

[DonggeLiu]

We should try this again.

Yep, let's also try using tools with the 2.5 Pro model.
It's less likely to cause the issues above and more likely to be used in the future.