Add golang custom label sample

[ConnorKirk]

Issue #, if available:

Description of changes: This PR adds a Custom Labels sample using the Go language

The sample demonstrates using the DetectCustomLabels API. It outputs an image with the bounding boxes drawn around the image under inspection.

There was an existing python sample under the custom-labels directory. I have moved this to a python subdirectory, and added a golang sibling directory.

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[ConnorKirk]

Good feedback, thank you. I will add a README