feat: Non-VRChat Avatars support

[kaikoga]

Discussed in https://github.com/anatawa12/AvatarOptimizer/discussions/522

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

Required Tasks

• [x] Add #if UNITY_2020_2_OR_NEWER
• [x] Add Version Define AAO_VRCSDK3_AVATARS against "com.vrchat.avatars"
• [x] #if AAO_VRCSDK3_AVATARS VRChat specific AAO components
• [x] #if AAO_VRCSDK3_AVATARS VRChat specific AAO passes
• [x] Use ndmf 1.2.0 API to reduce VRCAvatarDescriptor usage (if any)
• [x] #if AAO_VRCSDK3_AVATARS VRCSDK component references within generic AAO components
• [x] Update code generators

Not right now (create another issue)

• UniVRM support: VRM0 / VRM1
  • Add Version Define AAO_VRM1 against "com.vrmc.vrm" and/or AAO_UNIVRM against "com.vrmc.univrm"
  • Add VRM related implementations to AAO, compile flags guarded
  • Add UniVRM component references to generic AAO components
    • Ideas here
  • Implement UniVRM specific AAO components and passes , if AAO is a better place
    • Ideas here
  • Other possible VRM specific optimization
    • Ideas here
• Other non-VRChat avatar supports (by someone)

[kaikoga]

Done?

[anatawa12]

Done?

Yes.

Thank you a lot for your hard work.