core: add CPUQuotaPeriodSec=

core: add CPUQuotaPeriodSec=

This new setting allows configuration of CFS period on the CPU cgroup, instead
of using a hardcoded default of 100ms.

Tested:
- Legacy cgroup + Unified cgroup
- systemctl set-property
- systemctl show
- Confirmed that the cgroup settings (such as cpu.cfs_period_ns) were set
  appropriately, including updating the CPU quota (cpu.cfs_quota_ns) when
  CPUQuotaPeriodSec= is updated.
- Checked that clamping works properly when either period or (quota * period)
  are below the resolution of 1ms, or if period is above the max of 1s.