feat: Non-VRChat Avatars support

[kaikoga]

Discussed in https://github.com/bdunderscore/modular-avatar/discussions/467

Wrap VRCSDK dependencies of Modular Avatars in compile flags to support non-VRChat avatars.

Required Tasks

• [x] Add #if UNITY_2021_2_OR_NEWER
• [ ] Add Version Define MA_VRCSDK3_AVATARS against "com.vrchat.avatars"
• [ ] #if MA_VRCSDK3_AVATARS VRChat specific MA components and passes
• [x] Use ndmf 1.2.0 API to reduce VRCAvatarDescriptor usage
• [ ] #if MA_VRCSDK3_AVATARS VRCSDK component references within generic MA components

Not right now (create another issue)

• UniVRM support: VRM0 / VRM1
  • Add Version Define MA_VRM1 against "com.vrmc.vrm" and/or MA_UNIVRM against "com.vrmc.univrm"
  • Add VRM related implementations to MA, compile flags guarded
    • Add UniVRM component references to generic MA components
      • Ideas: Some PhysBones specific logics might also need to be able to handle SpringBones
    • Implement UniVRM specific MA components and passes , if MA is a better place
      • Ideas here
    • Other possible VRM specific optimization
      • Ideas: VisibleHeadAccessory can make use of VRMFirstPerson
• Other non-VRChat avatar supports (by eager somebody)