Add ability to choose an initial free minting amount on fixed price contracts

[IronGiu]

Hello, can be a good (and maybe esay to implement) feature to give ability to choose an amount that can be free minted before the fixed price minting, for example: on a collection of 10k NFT i can choose to make first 1000 free to mint, then the other 9000 at a fixed price of 0.001!