Add label_colors argument to draw_bounding_boxes

[carandraug]

Initial discussion and rational for this feature is in issue #8317 . As mentioned on that issue, this first PR addresses only the issue of label colour.

On issue #8317, I proposed to implement this by having a draw_label_kwargs argument which could then be passed to draw.text. This would exposes all the flexibility provided by PIL (font stroke width, separate font fill and stroke colours, etc). However, after doing that, I think that this "simpler", albeit more limited, approach is better because my original proposal:

1. messy when there are multiple boxes - we can pass a list of colours for the boxes and the same should be supported for the label colour. If we require a dict of kwargs then we would need a list of kwargs, many of them duplicated. It's just messy.

2. would bind us to PIL - if in the future this is done with something other than PIL then it would be difficult to keep BC

3. user needs to reference PIL documentation - not only the documentation is now elsewhere, different PIL versions support different arguments so it can be tricky for the user to know what arguments are supported

[pytorch-bot]

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