Allow Promo Assignment to specific Buyers

[abishop451]

As a Marketplace Owner, I would like to create a promo and make it eligible only for a certain Buyer in my organization.

Currently, Headstart only allows that the promo is available to ALL Buyers. I'd like to also make the "specific buyers" eligibility selection available. We can skip the "Specific buyer group" eligibility at this time.

If possible, please allow multi-select so a promo can be assigned to multiple selected Buyers.