Call the new `ConnectedAppService` DTD methods when VM Service instances are started / stopped

[kenzieschmoll]

This issue tracks the IntelliJ work required for dart-lang/sdk#60540.

What needs to be done?

• Call ConnectedApp.registerVmService when a vm service instance is started
• Call ConnectedApp.unregisterVmService when a vm service instance is shut down

As a reference the parallel VS code work was addressed in Dart-Code/Dart-Code#5527.

Why?
This will give DTD clients the ability to fetch all the active VM services (active debug sessions). Without the ConnectedApp service, this was only possible if DTD was started from VS Code, where the Editor.getDebugSessions method is registered.