Add ability to "turn off" KGX's prefix mapping functionality

[sierra-moxon]

In the transform of various inputs (like obo files in KG-obo), we have the need to provide some IRI munging in order to normalize prefixes.

There are a few upstream sources of prefix maps:
github.com/linkml/linkml/prefixmaps (though this requires a bijective map, and sometimes we don't have those maps yet), which builds on the curies package (which uses bioregistry under the hood).

We'd like the ability to "turn off" KGX's munging of prefixes -- this may actually lead to a change upstream in biolink or linkml.