[Feat]: Dart now has an SDK

### Is your feature request related to a problem? Please describe.

Just to inform you I have put together a Dart SDK, its a straight port of the JS SDK to Dart with some additional executor construction helper functionality.

It can be found [here](https://pub.dev/packages/a2a) in the Dart pub package repository.

### Describe the solution you'd like

Suggestion : maybe you could add a list of third party SDK's to the ones listed in the README, the more SDK's potential users can see the better.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The issue asks to add a third-party Dart SDK to the README. Look at the README file in the repository root to see how other SDKs are listed. Add a bullet point or section for the Dart SDK with a link to https://pub.dev/packages/a2a. Verify the formatting matches existing entries and submit a pull request.

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