Utility function to add content tags to classes by classpath string

[budak7273]

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