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

## Desired behavior
Add a `` tag under the `` tag in [SDF 1.8](http://sdformat.org/spec?ver=1.8&elem=world#world_include) to override the included model's cast_shadows values.

## Alternatives considered
You could apply 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 ``. 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.

The issue points to the SDF 1.8 world include element and uses as the analogy; start by tracing how include elements and boolean values are parsed and represented. Done means supporting an optional value under , defaulting it to 1, and adding coverage for overriding the included model's shadow behavior.

Written by the indexing model from the issue text.