Auto-generate `id` prop in `GlFormCheckbox `

As @ealcantara noted in an MR:

Setting id ensures that the GlFormCheckbox component will add the for attribute to the checkbox’s accompanying label. We need to do this to provide a human readable accessibility name to checkboxes.

We should not rely on each implementation to add this prop if it's an accessibilty feature. Since this can easily be avoided, I'd suggest to add to the component an id prop with a default value of _.uniqueId()

accessibility