Onboard SIG: Dart & Flutter

### SIG Name

SIG Dart & Flutter

### Assignee

svrnm

### Project Proposal PR

https://github.com/open-telemetry/community/pull/3517

### Onboarding Checklist

- [ ] Project Board
- [ ] Add Meeting Times, GC Liaison, Slack Channel, Google Doc to workstreams.yml
- [x] SIG Slack Channel
- [x] GitHub Teams