Add thumbhash encoder

[slam]

The c++ implementation of thumhash encoder is based on the rust reference implementation found here:

https://github.com/evanw/thumbhash/blob/main/rust/src/lib.rs

We modified it in the following ways:

• Make it work with OpenCV mat as input frame
• Handle images with or without an alpha channel
• Handle grayscale images
• Perform simple downsampling of images. We don't need very many pixels to get a good hash.

[slam]

@brian-armstrong-discord High level questions I have:

• Are we wiring up at the correct level of abstraction?
• Is this efficient in terms of golang garbage collection?

I modeled this somewhat after the existing non-opencv encoders. Definitely open to suggestions.