Allow PartsRepresentable to throw errors

[morganchen12]

This is a breaking change. See the internal proposal for more details.

[paulb777]

What's an API example that will break and need to be changed?

[morganchen12]

See this file: https://github.com/google/generative-ai-swift/pull/88/files#diff-a61268065794a7fb215fc6d6e84f0127f200a881647fe588e9b06477683d4a9b

I left this PR as a draft because I think we should do some more formal API proposal before baking it in. We can discuss in standup tomorrow.

[peterfriese]

Some questions around showing how to use the updated APIs, and some remarks around naming.

[peterfriese]

LGTM, with one final nit: use a consistent name for the autoclosure.

Places using content:

https://github.com/google/generative-ai-swift/blob/d962763ca1095879b54f2c12a9c0d3e708358aac/Sources/GoogleAI/Chat.swift#L91
https://github.com/google/generative-ai-swift/blob/d962763ca1095879b54f2c12a9c0d3e708358aac/Sources/GoogleAI/Chat.swift#L44
https://github.com/google/generative-ai-swift/blob/d962763ca1095879b54f2c12a9c0d3e708358aac/Sources/GoogleAI/GenerativeModel.swift#L114
https://github.com/google/generative-ai-swift/blob/d962763ca1095879b54f2c12a9c0d3e708358aac/Sources/GoogleAI/GenerativeModel.swift#L259

Places using contentClosure:

https://github.com/google/generative-ai-swift/blob/d962763ca1095879b54f2c12a9c0d3e708358aac/Sources/GoogleAI/GenerativeModel.swift#L172