Proposal: Add <cast_shadows> tag under the <include> tag

[ddengster]

Desired behavior

Add a <cast_shadows> tag under the <include> tag in SDF 1.8 to override the included model's cast_shadows values.

Alternatives considered

You could apply <cast_shadows> under the visuals of the included model but that would apply to all usages of the model.

Implementation suggestion

Value should be 0 or 1, similar to <static>. Default to 1.

Additional context

The need for this feature sprung from problems in lightmapping models. Models with lightmaps baked into them would render both the lightmapped shadows and the ignition light's shadows.

[chapulina]

Here's a more generic version of this issue: #128

I think it would be more valuable to have a generic method for overriding any tag than adding tag overrides as we need them.