Compatibility shim for usdchecker-style rules to Validation Framework

[asluk]

All the momentum around migrating usdchecker-style rules to Validation Framework is great, but having a compatibility shim to be able to run unmigrated rules would be great, especially as we have implemented a large set of rules for https://docs.omniverse.nvidia.com/kit/docs/asset-validator/latest/index.html . This would also accelerate the ability to deprecate usdchecker itself without being blocked on all rules being migrated to the Validation Framework.

Thanks!

[jesschimein]

Filed as internal issue #USD-10320