[Unreal] Add the ability to map physical materials to Steam Audio Materials

[JohnMega]

Adding a Steam Audio Geometry component to an actor requires that you pick one of the Steam Audio Materials for that actor, unless you want to use the default material. This makes the "Add Steam Audio Geometry Component to all actors" less useful, because you still need to set the material for every actor. The thing is, in Unreal we have already set physical materials for all of the actors. It feels silly to have to say "this mesh is brick, and hey Steam, this mesh is brick".

This PR adds a TMap PhysMatToSteamAudioMatTable to the plugin settings, where the Key is the physical material and the Value is the Steam Audio Material.

This PR also adds the ability to select a "default" Steam Audio Material when adding a Geometry Component to All Actors. This means that for all actors whose physical materials do not match the PhysMatToSteamAudioMatTable, "default" Steam Audio Material will be created.

There were also some requests on the forums to add this feature:
https://steamcommunity.com/app/596420/discussions/0/6015206719888268562/
https://steamcommunity.com/app/596420/discussions/0/595145594903778702/

Media

Video demonstrating the feature:

feature.mp4

[lakulish]

@JohnMega Going to merge this shortly, but wanted to leave an additional note here. Now that this change adds a mapping from physics materials to Steam Audio materials, we could just completely get rid of the Material field on the Steam Audio Geometry component, and modify the export code to always look up using the mapping.