[infra] Add FFIgen and package:objective_c to pub workspace

[dcharkes]

WIP:

• [infra] add ffigen, objective_c, ffi to workspace #2834

With these packages added to the pub workspace, all the samples and integration tests that use both hooks and and FFIgen will always resolve to tip of tree instead of via pub.dev.

Follow up:

• [infra] Roll FFIgen and package:objective_c into g3 via DEPS in Dart SDK #2862