In fact, it's partially there already. When view_builder::start() is called is first calls initialization code (the start_in_background() method), then kicks do_build_step() that runs a background fiber to perform build steps. The starting code inherits scheduling group from main(). And the step fiber code needs to run itself in a maintenance scheduling group, so it explicitly grabs one via database->db_config.
This PR mainly gets rid of the call to database::get_streaming_scheduling_group() from do_build_step() as preparation to splitting the streaming scheduling group into parts (see SCYLLADB-351). To make it happen the do_build_step() is patched to inherit its scheduling group from view_builder::start() and the start() itself is called by main from maintenance scheduling group (like for other view building services).
New feature (nested scheduling group), not backporting
Closesscylladb/scylladb#28386
* github.com:scylladb/scylladb:
view_builder: Start background in maintenance group
view_builder: Wake-up step fiber with condition variable
ec6a2661de