cmd/sync: support existing and ignore-existing flag

[zhijian-pro]

close #3559

[codecov-commenter]

Codecov Report

Patch coverage: 21.42% and project coverage change: -0.07 ⚠️

Comparison is base (fdd216a) 56.23% compared to head (b539b23) 56.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3566      +/-   ##
==========================================
- Coverage   56.23%   56.17%   -0.07%     
==========================================
  Files         149      149              
  Lines       36762    36781      +19     
==========================================
- Hits        20674    20661      -13     
- Misses      13826    13850      +24     
- Partials     2262     2270       +8     

Impacted Files	Coverage Δ
pkg/sync/config.go	0.00% <0.00%> (ø)
pkg/sync/sync.go	43.03% <0.00%> (-0.49%)	⬇️
cmd/sync.go	51.94% <100.00%> (+1.44%)	⬆️

... and 5 files with indirect coverage changes

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