Adds two new samples that use the Google Generative AI SDK for Dart.
Both were created by @rodydavis. I'm merely the uploader so to speak.
## Pre-launch Checklist
- [X] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [X] I signed the [CLA].
- [X] I read the [Contributors Guide].
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] All existing and new tests are passing.
