Add Steam Audio 4.8.1 spatializer plugin

Official Unity package under Assets/Plugins/SteamAudio; enabled as
Spatializer + Ambisonic Decoder in the audio settings (HRTF: sounds
behind the listener are filtered/muffled directionally). Download
staging Tools/SteamAudio/ gitignored.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Plugins/SteamAudio/Scripts/Runtime/SOFAFile.cs

@@ -0,0 +1,38 @@
+//
+// Copyright 2017-2023 Valve Corporation.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+using UnityEngine;
+
+namespace SteamAudio
+{
+    /*
+     * Represents a serializable object that is created when importing a .sofa file as an asset.
+     */
+    public class SOFAFile : ScriptableObject
+    {
+        // The name of the sofa file that was imported.
+        public string sofaName = "";
+
+        // The imported data.
+        public byte[] data = null;
+
+        // The volume correction factor in dB.
+        public float volume = 0.0f;
+
+        // Volume normalization type.
+        public HRTFNormType normType = HRTFNormType.None;
+    }
+}