Tracking bug: Changes to the Dart VM service interface should be easier to roll out across `vm_service` and `dwds`

[elliette]

Tracking bug for go/dwds-vm-service-proposal:

To address Issue #1 (It is easy for developers to accidentally make changes to package:vm_service that end up breaking web debugging workflows):

• During development, package:dwds will depend on package:vm_service as a git dependency override (see: Add vm_service as git package dependency #2078).
• Any breaking changes of the VM Service interface will be caught during development of package:dwds due to the dependency override. We should also add a daily cron job to further detect this.
• When package:dwds is ready to be released, the dependency override will need to be removed (see: Consider pubspec_overrides.yaml when publishing pub#3782).

To address Issue #2 (Unblocking Dart SDK rolls into google3 after a change to the Dart VM Service interface is a painful process):

• Roll package:dwds into google3 with the Dart SDK

[bkonyi]

Handled by introducing package:vm_service_interface and using the VM service packages as Git dependencies.