Add: generic resource manager

[Razish]

e.g. registering shaders, textures, meshes, materials.

Templated, inherited or handle-based?
std::shared_ptr to indicate ownership? manual ref-count? resources will be shared

need to discern between networked resources and local resource handles, i.e. server may indicate a material is being used on an entity, which correlates to multiple resources on the client?