[flutter_tools] New feature: implement `flutter announce` sub-command to show announcements in terminal

If possible, we should leverage the same code the IDE plugins/devtools does: https://github.com/flutter/devtools/blob/551a603fc551b7022cc249493c91011d64b1ed81/packages/devtools_app/lib/src/shared/release_notes/release_notes.dart
https://github.com/flutter/devtools/blob/374f37db70ab18acf1055807c4f59d14e85d24ce/packages/devtools_app/lib/src/shared/survey.dart#L17