Add a simple background tasks API for storage migrations

api/custom/datastores.go

api/custom/tasks.go

api/webserver/webserver.go

...rollers/maintenance_controller/maintainance_controller.go

docs/admin.md

migrations/10_add_background_tasks_table_down.sql

migrations/10_add_background_tasks_table_up.sql

storage/stores/metadata_store.go

types/background_task.go