[Firebase AI] Add thought summary and signature support

[andrewheard]

WIP - Added support for including thought summaries and thought signatures.

Manually tested through the quickstart but still needs unit tests and deciding if the approach in getText makes sense. Chat handling will also need to be implemented.

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[andrewheard]

cc: @dlarocque

[hsubox76]

Maybe instead of passing a specific boolean for thoughts we pass through a filter function here:

partFilter: () => boolean

and then when we call it in addHelpers() above it's like getText(response, (part) => !part.thought) for text() and it's getText(response, (part) => part.thought) for thoughtSummary(). And then in the condition below it's if (part.text && partFilter(part))

This makes it read clearly when it's called (or we can just add a comment before true and false). It might get unwieldy if we have other kinds of text parts in the future that have more complex logic though.