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.
* make alert saved object type hidden
* fix support for hidden alert type in alerting tests
* updated api docs
* fixed some missing types and unused imports
* fixed test broken by field rename
* added support for including hidden types in saved objects client
* fixed merge conflict
* cleaned up some test descriptions
* adds a getClient api to Encrypted Saved Objects
* fixed alerts fixture
* added missing plugin type in alerting
* removed unused field
* chaged ESO api to an options object as per Security teams request
* fixed usage of eso client
* fixed typos and oversights
* split alerts file into two - for actions and alerts
