Allow Open minting with HTS

[0xJepsen]

Currently, when you create an NFT, you are required to set a supply key that corresponds to the key authorized to mint tokens of the specified NFT type. The threshold key can be adopted if you want to make minting available to select individuals (for example, an exclusive prelaunch). However, there is currently no mechanism to allow permissionless minting for anyone.

Permissionless minting is a considerable part of the current NFT ecosystem. I Prepose we make that Supply key field optional, and if left empty, then any key should be qualified to mint as long as the current supply is below the max reserve.