Add a new getIsolatePauseEvent method in the VM service.

In Flutter hot reload, the flutter_tools are calling the `getIsolate`
to read the pause state of the isolate, which returns a full list of
libraries loaded in the isolate. The size of the return object grow
linearly to the number of Dart files, which can take ~500ms to transfer
for a large app.

This change adds a new method to only read what is needed.

TEST=pkg/vm_service/test/get_isolate_pause_event_rpc_test.dart

Change-Id: If19910cb3ff5d5057932551ac738afd3c3136fac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/305362
Reviewed-by: Ben Konyi <bkonyi@google.com>
Auto-Submit: Chingjun Lau <chingjun@google.com>
Commit-Queue: Derek Xu <derekx@google.com>