External DevTools instances can be registered with DDS at runtime, allowing for DDS
to redirect DevTools requests to the external DevTools server.
TEST=pkg/dds/test/external_devtools_instance_test.dart
Change-Id: I0bed34029b6ea7d935f77a031ff99b73b986b068
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/278663
Reviewed-by: Kenzie Davisson <kenzieschmoll@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>