Count subquery

[andyfengHKU]

Add count subquery. This should also fix issue #2182

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (cd61648) 90.71% compared to head (6ee73d2) 90.68%.

Files	Patch %	Lines
src/planner/plan/plan_subquery.cpp	81.48%	5 Missing ⚠️
...c/processor/operator/hash_join/hash_join_probe.cpp	84.61%	2 Missing ⚠️
...inder/bind_expression/bind_subquery_expression.cpp	95.45%	1 Missing ⚠️
src/parser/expression/parsed_expression.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2429      +/-   ##
==========================================
- Coverage   90.71%   90.68%   -0.03%     
==========================================
  Files        1023     1023              
  Lines       36948    37014      +66     
==========================================
+ Hits        33517    33567      +50     
- Misses       3431     3447      +16     

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