Add a way to set TF_DEBUG variables in the render settings

**Describe the feature you'd like**
We want to be able to set the TF_DEBUG variable in a scene or as argument of the render driver. This way we can launch renders with more interesting logs without having to change the local environment variable TF_DEBUG. This should be possible by using the TfDebug::SetDebugSymbolByName function.

Start by locating the render settings and render driver entry points, then read how they currently handle environment configuration. Check the documented TfDebug::SetDebugSymbolByName function and define completion as allowing TF_DEBUG values to be set per scene or render-driver argument without changing the local environment.

Written by the indexing model from the issue text.