Per-capability spend caps for GitLab Flex

With GitLab Flex, overage from one capability draws down the same commitment every other capability depends on. You can now set a spend cap for each capability when you adjust your Flex reservation in the Customers Portal: Restricted (no overage past the reservation), Usage cap (a bounded amount of overage), or Unlimited (unlimited overage past the reservation). When a capped capability reaches its cap, its usage stops for the rest of the billing period, and capabilities without a cap keep running.