future-utils: introduce with_scheduling_group
future-utils: introduce with_scheduling_group

Given a function and a scheduling group, convert it to a function that will
run in its correct task queue. This is useful when passing lambdas to
non-scheduling_group enabled functions.

dreamsxin/seastar,avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar