Is your feature request related to a problem? Please describe.
Currently only one item (or none) can be specified as the unlock item.

Describe the solution you'd like
Replace config option unlockSlotItem with an item tag for all items that can be used to unlock slots.

Describe alternatives you've considered
An alternative would be to change parsing of the config option to support some kind of list format. I do think tags are better for this, as the tag system is already in place and very much suitable for configurable registry object lists.

Additional context
A drawback of tags would be the lack of backwards compatibility with the previous configuration method. It is also less readily changeable by the end user, as creating or changing a datapack for it takes slightly more effort than changing the config file. I don't expect the latter to be a significant problem as the unlock item is unlikely to be changed often. With the first drawback, it would be appropriate to make this change when backwards-compatibility would otherwise naturally be broken.
There also needs to be a place where the item tag is documented for potential mod users to see, likely the curseforge page where similar things are currently described.