Copy to csv rework

[acquamarin]

This PR improves the existing copy-to-csv:

1. Allow copy to csv to be executed in parallel.
2. Avoid unnecessary flattens on copy to vectors.

E2E performance on copy ldbc-100 to csv file:
60452.38ms (M1 Max, 32GB RAM)

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (d81c628) 89.69% compared to head (4bed782) 89.69%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2242   +/-   ##
=======================================
  Coverage   89.69%   89.69%           
=======================================
  Files        1012     1013    +1     
  Lines       35862    35846   -16     
=======================================
- Hits        32167    32153   -14     
+ Misses       3695     3693    -2     

Files	Coverage Δ
src/common/serializer/buffered_serializer.cpp	92.85% <100.00%> (ø)
src/common/type_utils.cpp	86.07% <100.00%> (+16.26%)	⬆️
src/common/types/types.cpp	90.82% <100.00%> (ø)
src/function/vector_cast_functions.cpp	80.45% <100.00%> (+1.61%)	⬆️
...rc/include/common/serializer/buffered_serializer.h	66.66% <100.00%> (+33.33%)	⬆️
src/include/common/type_utils.h	100.00% <ø> (ø)
src/include/common/types/types.h	100.00% <ø> (ø)
src/include/function/cast/cast_functions.h	99.08% <100.00%> (-0.02%)	⬇️
src/include/function/cast/vector_cast_functions.h	95.65% <ø> (ø)
...rc/include/processor/operator/persistent/copy_to.h	100.00% <100.00%> (ø)
... and 9 more

... and 17 files with indirect coverage changes

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[andyfengHKU]

Take a look at coverage. I feel it should be covered but the report says no.