Fix T70298: FBX IO: Apply squared crease transform when importing/exporting.

Blender maps crease sharpness from internal [0, 1] to OpenSubdiv's
[0, 10] by squaring the value (see `get_edge_sharpness()`).

Other software seems to treat FBX crease as linear times 10 using OpenSubdiv.

This commits attempts to make FBX exported by Blender consistent with the
results from FBX exported from Maya regarding crease intensity.

Differential Revision: https://developer.blender.org/D5930