This PR adds `String? identifier` to `Semantics` and `SemanticsProperties`. The `identifier` will be exposed on Android as `resource-id` and on iOS as `accessibilityIdentifier`.
Mainly targeted at #17988
Initial Engine PR with Android support: https://github.com/flutter/engine/pull/47961
iOS Engine PR: https://github.com/flutter/engine/pull/48858
### Migration
This change breaks the SemanticsUpdateBuilder API which is on the Framework<-->Engine border. For more details see [engine PR](https://github.com/flutter/engine/pull/47961).
Steps:
part 1: [engine] add `SemanticsUpdateBuilderNew` https://github.com/flutter/engine/pull/47961
**part 2: [flutter] use `SemanticsUpdateBuilderNew`** <-- we are here
part 3: [engine] update `SemanticsUpdateBuilder` to be the same as `SemanticsUpdateBuilderNew`*
part 4: [flutter] use (now updated) `SemanticsUpdateBuilder` again.
part 5: [engine] remove `SemanticsBuilderNew`
948523b80c