Refactor glTF BlendShape animation

[GuoLei1990]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

Related issues: KhronosGroup/glTF#1768

[luzhuang]

+1

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (98ead75) 66.67% compared to head (48a34c8) 66.71%.

❗ Current head 48a34c8 differs from pull request most recent head 1221915. Consider uploading reports for the commit 1221915 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev/1.2    #1868      +/-   ##
===========================================
+ Coverage    66.67%   66.71%   +0.04%     
===========================================
  Files          472      472              
  Lines        23800    23799       -1     
  Branches      3394     3393       -1     
===========================================
+ Hits         15868    15878      +10     
+ Misses        6743     6731      -12     
- Partials      1189     1190       +1     

Files	Coverage Δ
packages/core/src/Entity.ts	85.58% <100.00%> (ø)
...es/core/src/animation/AnimationClipCurveBinding.ts	80.95% <100.00%> (+0.95%)	⬆️
packages/core/src/animation/Animator.ts	64.31% <100.00%> (+1.45%)	⬆️
...nternal/animationCurveOwner/AnimationCurveOwner.ts	42.50% <66.66%> (+1.12%)	⬆️
...r/BlendShapeWeightsAnimationCurveOwnerAssembler.ts	62.50% <0.00%> (+17.04%)	⬆️
packages/core/src/animation/AnimationClip.ts	39.62% <55.55%> (-3.24%)	⬇️
...ages/loader/src/gltf/parser/GLTFAnimationParser.ts	58.19% <69.23%> (-1.98%)	⬇️

... and 3 files with indirect coverage changes

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