How can I get/set sidebearing of layer?

I’m having troubles getting LSB and RSB of layer:

if I do:
```
font = GSFont()

for glyph in font.glyphs:
for layer in glyph.layers:
print (layer.LSB)
```

I get that GSLayer doesn’t have LSB attribute.

How can I change sidebearings of glyphs?

Start by inspecting the GSLayer API and the existing glyphsLib usage around layer metrics. No file, test, or entry point is named in the issue, so first identify where sidebearings are represented and how related properties are exposed. Done means documenting the supported way to read and change LSB and RSB, with a small usage example.

Written by the indexing model from the issue text.