type aliases for public api

per https://github.com/axodotdev/axoasset/pull/2#discussion_r1066591957 via @Gankra

the api is rather stringly typed, for reasons i think are fair, but it would be useful for us internally to use type aliases to make clear what we're doing

Start by inspecting the repository's public API and the linked pull-request discussion to identify the stringly typed values this issue refers to. Define internal type aliases that clarify their use while preserving the intended API behavior; done means the aliases cover the agreed cases and the existing project checks still pass.

Written by the indexing model from the issue text.