Ensure that the implementation of controllers supports control plane migration

[stoyanr]

How to categorize this issue?

/area control-plane-migration
/kind task
/kind test
/priority normal

What would you like to be added:

• Ensure that the implementation of this provider extension's controllers supports control plane migration properly, see https://github.com/gardener/gardener/blob/master/docs/extensions/migration.md.
• Test control plane migration manually with this provider extension as described in https://github.com/gardener/gardener/blob/master/docs/usage/control_plane_migration.md. You can also use the code of the existing integration test to trigger and test the migration in a more automated way, see https://github.com/gardener/gardener/blob/master/.test-defs/MigrateShoot.yaml.
• To enable the control plane migration, the SeedChange feature gate in Gardener API server should be switched on. Currently this is the case in a locally running gardener-apiserver, see https://github.com/gardener/gardener/blob/master/hack/local-development/start-apiserver

Why is this needed: