use ai fetch and remove types from this package

[cfortuner]

Updates this package to use https://github.com/dexaai/ai-fetch

As such, this pr now reexports the core types from ai-fetch.

NOTE: need to publish ai-fetch before we can update the package json here.

[rileytomasek]

it's exciting to see this happening ✨.

if ai-fetch is going to export the full openai-based types, shouldn't we be able to use them directly in the openai-client and delete openai-types/* and the openai dependency from this package? the current way feels duplicative, will run into issue matching type versions, and uses the openai package types directly instead of openai-types/*, which will risk bundler issues with the gnarly openai package.

[rileytomasek]

also, added @transitive-bullshit to the reviewers :)

[transitive-bullshit]

this seems to complicate things quite a bit; when the whole reason openai-fetch exists is to simplify the openai package.

if we're going with this route, I'd prefer a monorepo for ai-fetch + *-fetch since changes across one will likely necessitate changes across each of them.