Feature Request: support different shininess values for multiple visuals in a model

[scpeters]

Background

A feature for applying a shininess value to visuals from an SDFormat file was added in #3213. In Gazebo11, the SDFormat world file is parsed by physics::World and communicated to the rendering::Scene over a transport interface. While it would have been simplest to add a shininess field to the relevant protobuf message, this would have caused ABI problems. As a workaround, an ignition service is created for each model that can be called by the Scene to retrieve the shininess of the first link of the first visual of that model (see this portion of World::LoadModel). Although shininess can be specified on a per-visual basis in SDFormat, it is limited by the transport service mechanism to only report one shininess value per model.

Feature request

Expand the functionality of the service interface so that the shininess of each visual in a model can be queried independently.

[scpeters]

See #3235