feat: implement audio backend specific audio managers

[acid-chicken]

Working on #705

This PR changes the audio management system to use a more modular approach. The main changes include:

• Deprecating the old osu.Framework.Audio.AudioManager class, which was a monolithic audio manager.
• Introducing a new osu.Framework.Audio.Manager.IAudioManager interface and a new osu.Framework.Audio.Manager.AudioManager abstract class, which are no longer dependent on BASS, will allow for easier work on other audio backends in the future (cc: #6002).
• Introducing a new osu.Framework.Audio.Manager.Bass.BassAudioManager abstract class, which will contain BASS-specific audio management logic.
• Introducing a new osu.Framework.Audio.Manager.Bass.BassPrimitiveAudioManager class, which is a concrete implementation of audio management using BASS, so it can be used as a drop-in replacement for the old osu.Framework.Audio.AudioManager class. WASAPI support is completely removed in this class.
• Introducing a new osu.Framework.Audio.Manager.Bass.BassWasapiAudioManager class, which is a concrete implementation of audio management using BASS with WASAPI support.

And the following tweaks:

• Removed BASS-specific logics from osu.Framework.Threading.AudioThread and moved them to BassPrimitiveAudioManager and BassWasapiAudioManager.
• The audio backend and/or device can now be configured via environment variables OSU_AUDIO_BACKEND and OSU_AUDIO_DEVICE.

Notable breaking changes

• The old osu.Framework.Audio.AudioManager class is no longer available, and new audio manager classes have some design differences.
  • The new audio manager identifies audio devices by their identifiers provided by concrete implementations, instead of using their names.
  • The new audio manager does not require resource stores in the constructor and instead provides a SetStore method to set the resource store after construction.
  • The new audio manager does not have the global mixer handle at abstract levels. Instead, concrete implementations which need a global mixer implement osu.Framework.Audio.Manager.IGlobalMixerProvide interface to provide the global mixer handle.

Known issues

• ~~Switching devices later using WASAPI causes the sound to stop playing. I've probably made a design mistake regarding global mixer handling.~~
• ~~When a debugger breakpoint occurs while using WASAPI, buffer updates stall, causing the last data in the buffer to loop indefinitely. This may surprise or frustrate you.~~

[peppy]

I haven't considered the new code splitting, structure, DI usage or anything. That's going to take hours / days of consideration.

Hard to say if any of this abstraction is required when wasapi was already working with the old structure.

[acid-chicken]

• Switching devices later using WASAPI causes the sound to stop playing. I've probably made a design mistake regarding global mixer handling.

This problem is fixed @ 1d0f32463bea65e73003d9059b2fafac612edd87.

[acid-chicken]

When a debugger breakpoint occurs while using WASAPI, buffer updates stall, causing the last data in the buffer to loop indefinitely. This may surprise or frustrate you.

This problem is also fixed @ 07b472791c5b025ad5385a77f934177880b9c785.