refactor(frontend/expr): unifiy to_expr and as_expr, remove redundent code

[Enter-tainer]

Condition::as_expr and Condition::to_expr are very similar, except for one is for borrowed value and the other one is for owned value. However, these two functions are implemented by copying and pasting code. This pr unifies the borrowed version and the owned version.

Checklist

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

[codecov]

Codecov Report

Merging #1816 (04b68c9) into main (76c22f6) will increase coverage by 0.01%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##             main    #1816      +/-   ##
==========================================
+ Coverage   70.84%   70.85%   +0.01%     
==========================================
  Files         610      610              
  Lines       79689    79684       -5     
==========================================
+ Hits        56453    56462       +9     
+ Misses      23236    23222      -14     

Flag	Coverage Δ
rust	70.85% <84.21%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
...c/frontend/src/optimizer/plan_node/batch_filter.rs	81.81% <0.00%> (ø)
src/frontend/src/utils/condition.rs	96.24% <0.00%> (+6.53%)	⬆️
src/frontend/src/expr/utils.rs	98.71% <72.72%> (-0.96%)	⬇️
src/frontend/src/expr/mod.rs	91.27% <100.00%> (+0.90%)	⬆️
...frontend/src/optimizer/plan_node/logical_filter.rs	97.79% <100.00%> (ø)
...c/frontend/src/optimizer/plan_node/logical_join.rs	84.99% <100.00%> (+0.08%)	⬆️
.../frontend/src/optimizer/plan_node/stream_filter.rs	87.87% <100.00%> (ø)

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[st1page]

LGTM