Fix camera parameter translations and simplify code

[jjeziorski]

Fix the translation of focus distance and simplify the handling of camera lens and aperture parameters. Enhance the retrieval of camera attributes by searching for requested parameters directly.

Value for focus distance was incorrectly assumed in inches, which is not consistent with docs

I'm not sure where logic for computing lens focal and aperture came from, but it was expensive, units were way off and gave wrong values for depth of field calculations. With proposed changes values are consistent with values exported by maya-usd.

I've also added proposed change to support custom attributes by checking camera plugs.

[lilike-adsk]

@jjeziorski have you had a chance to try the change with custom attributes support?