Add OpenAI provider

[davidmigloz]

This PR adds support for OpenAI.

Not sure if you accept 3rd party providers. If not, this PR can be used as a reference for people to integrate OpenAI.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[domesticmouse]

Please run dart format

[davidmigloz]

fixed

[domesticmouse]

Hey @davidmigloz, sorry to do this to you. Could you please build a single merged version of these three PRs? We want to see that all three can be successfully merged together into main. Thanks!

/cc @mit-mit @csells

[davidmigloz]

Done! Closing this one in favor of #30