SG4 Spec: Stargaze Minter Contracts

A minter must store the factory that created it, so it can query its parameters:

pub struct MinterConfig<T> {
    pub factory: Addr,
    pub collection_code_id: u64,
    pub extension: T,
}

Custom minters can add more fields using extention.

A minimum, Stargaze minters should specify at least one Mint {} operation that takes 0 to many arguments.

pub enum ExecuteMsg {
    Mint {},
}