[Unity] Improve perfomance in FMOD + Steam Audio

[Enderlook]

Improve perfomance by reducing memory allocations per frame and GC pressure when using FMOD engine.

In an scene test of mine, I had 158 SteamAudioSources. Without this commit, SteamAudioSource.Update() took 3.77 ms per frame (7.2% of CPU budget) and allocated 137 kb per frame. After this commit, it took 2.8 ms per frame (5.8%) and allocates 79.8 kb per frame. That is a ~25% perfomance improvement in CPU and ~41% less of garbage for the GC.

For further improvement reflection should be eliminated (not shown in this commit). With my own tests I figured out it can reduce time to 0.17 ms (0.3%) and no garbage, equivalent to ~95% perfomance improvement. I did this by creating a new script without reflection (I can pass if you want) and then instantiating it with reflection (so reflection is only using at creation rather per update). However this script is outside Steam Audio library which is why it can have a reference to both Steam Audio and FMOD content. How to do this inside Steam Audio remains as a question...