Add a target for the Unity Asset Store #581

Today we upload our Unity SDK to the Unity's Asset Store "manually". It requires a plugin on Unity itself and there's some "magic" happening under the hood.

We'd need to reverse engineer that chatter and replicate that via a craft target so we can keep the marketplace up-to-date without having to manually do it.

To be clear, this isn't the same as UPM, which was added here:

• feat: Add upm target #209