Missing GetAmbientVolume in C# wrapper

[TheShadowKing19]

Describe the bug
The C# version of addon is missing GetAmbientVolume function, even though docs mention it and is present in GDScript version of the addon.

Affected version

• Addon version: 2.6.2
• Godot version: 4.2.2.stable.mono

To Reproduce
Steps to reproduce the behavior:

1. Create scene
2. Attach C# script to the scene
3. Try to call SoundManager.GetAmbientVolume()

Expected behavior
Returned variable of type float, holding Ambient Volume.