Solana: adding support for minting via SPL multisig program

[nik-suri]

This would enable multiple minters on Solana. For example, asset issuers could give the NTT Manager the ability to burn/mint tokens while still retaining their own ability to burn/mint tokens via their custom asset issuance process.

We could implement this via two separate instruction implementations rather than shoehorning both behaviors into one.