Implement union-loader

[acquamarin]

This PR implements the very basic loader for union dataType.
DDL to define a union column: UNION(FIELD1 TYPE1, FIELD2 TYPE2)
Rules for union column in csv:
User simply needs to write the field value in the corresponding union column. The csv loader will set the value to the union field where a casting is possible. If the casting from the value to multiple struct fields are defined, the csv loader will choose the field with smallest struct_field_idx.
Currently, the csv reader can only cast primitive types to union (BOOL, INT64, INT32, INT16, DOUBLE, FLOAT, DATE, TIMESTAMP).

[codecov]

Codecov Report

Patch coverage: 80.67% and project coverage change: +0.04% 🎉

Comparison is base (4a12985) 89.73% compared to head (f906657) 89.78%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1907      +/-   ##
==========================================
+ Coverage   89.73%   89.78%   +0.04%     
==========================================
  Files         869      869              
  Lines       31012    31103      +91     
==========================================
+ Hits        27828    27925      +97     
+ Misses       3184     3178       -6     

Files Changed	Coverage Δ
src/include/common/types/date_t.h	100.00% <ø> (ø)
src/include/common/types/timestamp_t.h	100.00% <ø> (ø)
src/include/storage/copier/struct_column_chunk.h	100.00% <ø> (ø)
src/include/storage/copier/table_copy_utils.h	100.00% <ø> (ø)
...age/in_mem_storage_structure/in_mem_column_chunk.h	47.36% <0.00%> (ø)
...e/in_mem_storage_structure/in_mem_column_chunk.cpp	32.93% <0.00%> (ø)
src/storage/store/node_column.cpp	79.77% <ø> (+0.92%)	⬆️
src/storage/copier/struct_column_chunk.cpp	69.59% <42.85%> (+3.94%)	⬆️
src/storage/copier/table_copy_utils.cpp	70.46% <48.78%> (-2.12%)	⬇️
src/common/type_utils.cpp	62.31% <80.00%> (-5.94%)	⬇️
... and 24 more

... and 19 files with indirect coverage changes

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