feat: adjust API

[ggicci]

1. Removed AdaptDecoder, AdaptDecoderFunc
2. Expose DecoderFunc[T]

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02% ⚠️

Comparison is base (e75f7bc) 96.46% compared to head (609b7ce) 96.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
- Coverage   96.46%   96.45%   -0.02%     
==========================================
  Files          21       21              
  Lines         623      621       -2     
==========================================
- Hits          601      599       -2     
  Misses         15       15              
  Partials        7        7              

Flag	Coverage Δ
unittests	96.45% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
decoder.go	96.77% <100.00%> (ø)
decoderadaptor.go	79.68% <100.00%> (-0.62%)	⬇️
file.go	100.00% <100.00%> (ø)

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