[agent-kit] Make Specialist and Looper Models switchable

[dglazkov]

TODO:

• [x] #2906
• [ ] Factor out "Big Model" board out of Specialist
• [ ] Implement 3.5 Sonnet as "Big Model" board
• [ ] ...

[dglazkov]

Currently, the model family that is used by Specialist and Looper is hard-coded to Gemini and the actual model is asked once per run (unless configured via inputs setting).

This is good, but ideally, we should offer a way to specify a model per component instance. For example, I should be able to say "run this Specialist with Gemini Flash, and run that Specialist with Gemini Pro" or even "run this Specialist with Claude 3.5 Sonnet".

Here's a sketch:

• There's a well-defined shape (inputs/outputs) of a "Model" board
  • There are likely multiple shapes for various kinds of models: for larger, multi-modal, and multi-features models like Gemini/GPT/Sonnet, for smaller models like Nano & Haiku, and for media models like DALL-E, etc.
• Board selector UI is somehow configurable to filter out and only accept boards of that shape (it might just be a model tag at first, similar to the tool tag)
• There's a new field on Specialist and Looper: Model. This field has Gemini Flash as default, but a user can click on the drop down and select any models that are available across the board servers they've signed on to.

Here are some mocks:

Default:

Drop-down open: