Support for proxy layers in loader-only functions

[pbalcer]

The layers in the loader are essentially adapters that wrap the underlying implementation to provide additional functionality, be it validation, tracing, or something else. Unfortunately, this breaks down when UR functions are not part of the adapter interface but instead are implemented in the loader. There's no place for the layers to hook into.

This issue is to devise and implement a way for the proxy layers to intercept loader-only functions.