[Span First]: Add new Span protocol

[buenaflor]

https://develop.sentry.dev/sdk/telemetry/spans/span-protocol/

[linear]

DART-249

[buenaflor]

• Dart: [Span First #2]: Add envelope type to Dart layer #3366
• Java: feat(envelope-item): Support span type sentry-java#4935
• Cocoa: No changes needed

All done, closing this issue