Add the prerequisite functionality in Dart `a2ui_core` to support the new `a2ui_agent` library

Prerequisite for: https://github.com/a2ui-project/a2ui/issues/2356

We need to make some changes to a2ui_core to make sure it provides all the necessary APIs that the new a2ui_agent library will need.

• for v0.9
• for 1.0