SG2 Spec: Terp Minter Factory Contracts

Common types needed for minter factories. New minter factories should extend these types to add their custom properties.

Every minter requires a factory that stores the minter's governance paramaters. A minter factory creates minters based on these paramaters.

Factories also restrict minters to a set of allowed terp721 code ids.

For example, the vending minter factory needs a shuffle_fee. This is added as a custom extension to MinterParams and UpdateParamsMsg.

pub struct VendingUpdateParamsExtension {
    pub shuffle_fee: Option<Coin>,
}
pub type VendingUpdateParamsMsg = UpdateMinterParamsMsg<VendingUpdateParamsExtension>;

pub struct ParamsExtension {
    pub shuffle_fee: Coin,
}
pub type VendingMinterParams = MinterParams<ParamsExtension>;

previously sg2