title: "pallet_broker::start_sales: Take `extra_cores` and not total cores"

doc:
  - audience: Runtime User
    description: |
      Change `pallet_broker::start_sales` to take `extra_cores` and not total cores.
      It will calculate the total number of cores to offer based on number of
      reservations plus number of leases plus `extra_cores`. Internally it will
      also notify the relay chain of the required number of cores.

      Thus, starting the first sales with `pallet-broker` requires less brain power ;)

crates:
- name: pallet-broker
  bump: minor