[REQ] add_builder should not reset up_vector and gravity

[jc211]

I don't know how others use add_builder but usually my "main builder" already has the correct gravity and up vector. For my use cases, I don't want add_builder to overwrite these.

[eric-heiden]

I think it would be good to at least make this behavior optional. There can be use cases where the user loads a USD model on the per-environment builder (child) which has a Z up vector and then adds this builder N times to the main builder (parent) which should receive the correct up axis from the child.