feat: implement nullif and coalesce function

[cykbls01]

What's changed and what's your intention?

PLEASE DO NOT LEAVE THIS EMPTY !!!

1. implement nullif function.

select nullif(expr1,expr2)

2. implement coalesce function.

select coalesce(expr1,expr2,expr3...)

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests

Refer to a related PR or issue link (optional)

#2283
#2284

[codecov]

Codecov Report

Merging #2308 (8adafb1) into main (151a2cf) will increase coverage by 0.03%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main    #2308      +/-   ##
==========================================
+ Coverage   71.02%   71.05%   +0.03%     
==========================================
  Files         676      677       +1     
  Lines       84568    84696     +128     
==========================================
+ Hits        60064    60181     +117     
- Misses      24504    24515      +11     

Flag	Coverage Δ
rust	71.05% <90.90%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
src/common/src/types/mod.rs	63.65% <0.00%> (-0.76%)	⬇️
src/expr/src/expr/mod.rs	48.83% <0.00%> (-1.17%)	⬇️
src/expr/src/expr/expr_coalesce.rs	95.18% <95.18%> (ø)
src/expr/src/expr/expr_case.rs	98.21% <100.00%> (+0.04%)	⬆️
src/frontend/src/binder/expr/function.rs	92.42% <100.00%> (+2.62%)	⬆️
src/frontend/src/expr/function_call.rs	99.25% <100.00%> (+<0.01%)	⬆️

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[neverchanje]

lgtm

[neverchanje]

Suggested change

	fn write_nullif_to_case_args(inputs: Vec<ExprImpl>) -> Result<Vec<ExprImpl>> {
	fn rewrite_nullif_to_case_when(inputs: Vec<ExprImpl>) -> Result<Vec<ExprImpl>> {