Load plugins from <visual><plugin>

Part of https://github.com/ignitionrobotics/ign-gazebo/issues/265

## Desired behavior

From https://github.com/ignitionrobotics/ign-gazebo/issues/265:

> Some more pointers for visual plugins that are purely related to rendering:

> * `ign-gazebo` should load ``s. As a reference, [here](https://github.com/ignitionrobotics/ign-gazebo/compare/fc15b5e5b6d5550d6317b02ded5f1fe6ce001300..6a74c3608c55ba7fbce856478f0836de568e4b51)'s what it took to load ``s.
> * Gazebo-classic's visual plugins are loaded by `gazebo::rendering` on both server and client scenes. I believe an equivalent implementation would have to live in `ign-rendering`.

## Alternatives considered

## Implementation suggestion

## Additional context