`reward_supply` should correspond to the reward's candy machine's `items_available` field

Currently the value has no invariant and it determines the supply for the whitelist token.

https://github.com/magicshards/breeding-program/blob/cbac8c39ed7c47a0f40b0dca2eb9f565b7f5fde9/programs/breed-program/src/lib.rs#L185

[CandyMachineData documentation](https://docs.rs/mpl-candy-machine/latest/mpl_candy_machine/struct.CandyMachineData.html#structfield.items_available)