Issue #2701277 by mikeryan: Use configuration entities as a registration mechanism for migrations

config/schema/migrate_plus.data_types.schema.yml

config/schema/migrate_plus.destination.schema.yml

config/schema/migrate_plus.process.schema.yml

config/schema/migrate_plus.schema.yml

config/schema/migrate_plus.source.schema.yml

...le/config/install/migrate_plus.migration.beer_comment.yml

...ample/config/install/migrate_plus.migration.beer_node.yml

...ample/config/install/migrate_plus.migration.beer_term.yml

...ample/config/install/migrate_plus.migration.beer_user.yml

migrate_plus.services.yml

src/Entity/Migration.php

src/Entity/MigrationGroup.php

src/Entity/MigrationInterface.php

src/Plugin/Discovery/ConfigEntityDiscovery.php

src/Plugin/MigrationConfigEntityPluginManager.php