arv-fake-gv-camera: support Mono 14 pixel formats

[Rz-Rz]

Is your feature request related to a problem? Please describe.
I have some code that expects to stream a camera, using Aravis, and to receive Mono 14 frames, converts them to BGR8 and outputs BGR8. I wanted to write some tests for that piece of code using arv-fake-gv-camera but I found that I cannot set the pixel format to Mono14 as I desired.

Describe the solution you'd like
I'd like for arv-fake-gv-camera to support Mono 14 pixel formats for testing code that expects to have to convert images.