refactor(frontend): rename Expr::to_protobuf to Expr::to_expr_proto

[TennyZhuang]

Signed-off-by: TennyZhuang zty0826@gmail.com

What's changed and what's your intention?

PLEASE DO NOT LEAVE THIS EMPTY !!!

Sometimes we need some raw proto message, e.g. InputRefExpr, it's better for each type to implement their own to_proto, and implement to_expr_proto to wrap them.

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)

[codecov]

Codecov Report

Merging #2049 (db9a889) into main (abb873a) will increase coverage by 0.00%.
The diff coverage is 53.33%.

@@           Coverage Diff           @@
##             main    #2049   +/-   ##
=======================================
  Coverage   70.95%   70.95%           
=======================================
  Files         632      632           
  Lines       81082    81089    +7     
=======================================
+ Hits        57529    57536    +7     
  Misses      23553    23553           

Flag	Coverage Δ
rust	70.95% <53.33%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/frontend/src/expr/agg_call.rs	42.85% <0.00%> (ø)
src/frontend/src/expr/correlated_input_ref.rs	80.00% <0.00%> (ø)
src/frontend/src/expr/subquery.rs	48.27% <0.00%> (ø)
...c/frontend/src/optimizer/plan_node/batch_filter.rs	77.14% <0.00%> (-4.68%)	⬇️
.../frontend/src/optimizer/plan_node/batch_project.rs	81.03% <0.00%> (ø)
...ontend/src/optimizer/plan_node/stream_hash_join.rs	60.00% <0.00%> (ø)
...ntend/src/optimizer/plan_node/stream_hop_window.rs	63.15% <0.00%> (ø)
src/frontend/src/expr/mod.rs	90.84% <57.14%> (ø)
src/frontend/src/expr/function_call.rs	95.16% <100.00%> (ø)
src/frontend/src/expr/input_ref.rs	92.64% <100.00%> (ø)
... and 5 more

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[skyzh]

Please run ./risedev check to fix.