Utility function to add content tags to classes by classpath string

Being able to add content tags to another mod's content without that mod in the editor is an intended but poorly supported use case. Currently the way to do that is to store the class paths as string, try to blocking load resolve them into classes yourself before the registry freeze (the easy BP async one could put it after freeze), then call the tag add helper function. If there is not an easier/better way, this could at least be put into SML so other mods don't need to reinvent the wheel.

Came up in discussion between Rob and Digby when making this mod: https://ficsit.app/mod/FreeArchitecture

The issue names no files or tests. Start by reviewing the linked FreeArchitecture mod and the current tag-add helper, then compare class-path resolution before registry freeze with the stated async case; done is an agreed SML utility or clearer supported path for adding tags by classpath string.