feat(expr): implementation for char_length operation

[nasnoisaac]

What's changed and what's your intention?

implement char_length operation

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

related issue #112

[codecov]

Codecov Report

Merging #2991 (b24f9f9) into main (109e7d0) will decrease coverage by 0.00%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##             main    #2991      +/-   ##
==========================================
- Coverage   73.25%   73.25%   -0.01%     
==========================================
  Files         730      730              
  Lines       98392    98399       +7     
==========================================
+ Hits        72080    72082       +2     
- Misses      26312    26317       +5     

Flag	Coverage Δ
rust	73.25% <45.45%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
src/expr/src/expr/expr_unary.rs	74.43% <0.00%> (-1.08%)	⬇️
src/expr/src/expr/mod.rs	48.88% <ø> (ø)
src/expr/src/vector_op/length.rs	100.00% <100.00%> (ø)
src/frontend/src/binder/expr/function.rs	79.47% <100.00%> (+0.27%)	⬆️
src/frontend/src/expr/type_inference.rs	99.00% <100.00%> (ø)
src/meta/src/hummock/mock_hummock_meta_client.rs	43.82% <0.00%> (-1.13%)	⬇️
src/storage/src/hummock/compactor.rs	69.86% <0.00%> (-0.18%)	⬇️
src/storage/src/hummock/local_version_manager.rs	84.12% <0.00%> (-0.16%)	⬇️
src/meta/src/hummock/hummock_manager.rs	86.96% <0.00%> (+0.10%)	⬆️
src/common/src/types/ordered_float.rs	24.90% <0.00%> (+0.19%)	⬆️
... and 1 more

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[lmatz]

nit:
By https://www.postgresql.org/docs/9.1/functions-string.html and https://www.postgresql.org/docs/current/datatype.html,
it seems the data type of the return value should be signed four-byte integer

[lmatz]

LGTM!