Introduces migrations into Encrypted Saved Objects.
The two main changes here are:
1. The addition of a createMigration api on the EncryptedSavedObjectsPluginSetup.
2. A change in SavedObjects migration to ensure they don't block the event loop.
As part of the work towards adding RBAC & Feature Controls support in Alerting (https://github.com/elastic/kibana/issues/43994), we've decided that the AlertsClient will handle authorisation instead of relying on the SavedObjectsClient on its own.
To prevent (or at least, minimise the chances of) bypassing this auth model by using the SavedObjects client this PR makes the `Alert` SavedObject type a _hidden_ type and given the AlertsClient permission to interact with it.
