Add plugin to add shaders to a visual

[chapulina]

Ignition Rendering provides API to set vertex and fragment shaders to visuals, see the custom shaders tutorial. It would be convenient if users could set shaders from their SDF files.

Desired behavior

The user can load a plugin attached to a visual (needs #265) that loads a shader. For example:

<visual name="vis">
  <plugin ...>
    <fragment>/path/to/file</fragment>
    <vertex>/path/to/file</vertex>
  </plugin>
</visual>

Alternatives considered

We could make this part of the SDF spec and populate components so the plugin gets the shader paths.

Implementation suggestion

This tutorial on accessing the rendering API from ign-gazebo plugins should help. We may need to create 2 plugins, one for the backend and one for the frontend.

Additional context

The ocean model would be a good example to load using this plugin.

[chapulina]

ShaderParam plugin added in #1275