[C API][Unreal] Add the ability to get closest Occluder Mesh

[JohnMega]

This PR added the ability to get closest Instanced occluder mesh information for a sound source. Essentially, we get information from the IPLHit structure, which is filled in when transmission is calculated in DirectSimulator. At the same time, an InstancedMeshIndex field was added to the SteamAudioDynamicObjectComponent, which indicates the index of the given Instanced Mesh at the scene (in the case of the received closest occluder mesh information, this index is equal to IPLHit::objectIndex).

Information about closest Instanced occluder mesh is obtained using the GetOutputs() function of SteamAudioSourceComponent.

There were also some requests on the forums about this feature: https://steamcommunity.com/app/596420/discussions/0/3826411948446325128