Support gazebo convention for texture folder in model directory

[mxgrey]

Gazebo models use a non-standard convention for where textures are placed in a model directory. Instead of being placed next to the relevant .obj / mesh file, they are placed in a materials/textures/ folder. This creates problems for loading these assets because asset loaders meant for conventional usage cannot find the textures.

Here's an idea for how we can make this work:

• Add a hook into bevy_obj that allows us to customize how textures are searched for
• Fetch model directory layout information from gz-fuel so we can search efficiently for the actual URI of the texture
• Fetch the texture

Another idea to consider is having bevy_obj support an option to use a fallback texture in cases where the texture file was not found.