Move KibanaMigrator into Server SavedObjectsService (#43433) (#47011)
* Rename SavedObjectsService -> SavedObjectsLegacyService
* Expose legacy pluginSpecs from Core LegacyService
* Expose legacy uiExports from Core LegacyService
* Move kibana config to NP
* Expose pluginExtendedConfig from LegacyService
* Make KibanaMigrator NP compatible
* KibanaMigrator -> NP SavedObjectsService
* SavedObjectsService never stop retrying ES connection error
* Move waiting for migrations to complete till after legacy service start
* Fix ESArchiver's KibanaMigrator
* Fix reload logging config tests
* Run migrations on savedobjects start
* Fix env tests
* Fix and make legacy tests more robust/isolated
* Cleanup code
* Fix invalid config test
* Fix SavedObject Migrations logging test
* SavedObjectsService tests
* Lifecycle logging and improve getting kibanaConfig instance
* Fix awaitMigration bug and test
* Fix typing error
* Review comments
* Remove unecessary KibanaConfig class
* Move legacy plugin config extension, specs, uiExports entirely into Core
uiExports, specs, disabledSpecs, config now get injected into KbnServer
* Fix config deprecation test
* Use existing logger mock
* Create SavedObjectsConfig for migration config
* Define KibanaMigratorContract type
* KibanaMigratorContract -> IKibanaMigrator + docs improvements
* Fix esArchiver's KibanaMigrator
* Fix plugin generator integration test
* ConfigServiceContract -> IConfigService
* Address review comments
* Review nits
* Document migrations.skip config
* Review comments continued...
* awaitMigrations -> runMigrations
* Type improvements
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