Support linear exposure scale in cameras

The "exposure" attribute in cameras should take into account the new `GetLinearExposureScale` function in `HdCamera` that provides a more meaningful value.
Also, a quick look at the usd translator code seems to suggest that we might not even support the exposure in cameras at all when rendering directly through usd, in which case this should be fixed

Start by tracing the camera exposure attribute and the USD translator code mentioned in the issue, then inspect how HdCamera.GetLinearExposureScale is exposed. Done means camera exposure uses the linear exposure scale and remains supported when rendering directly through USD.

Written by the indexing model from the issue text.