Add:Support MySQL CHAR_LENGTH function.#501

[gongna-au]

What this PR does:

Which issue(s) this PR fixes:

Fixes #501

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Base: 36.53% // Head: 36.55% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (b71e2e4) compared to base (a545ceb).
Patch coverage: 72.72% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #544      +/-   ##
==========================================
+ Coverage   36.53%   36.55%   +0.02%     
==========================================
  Files         167      170       +3     
  Lines       21716    21785      +69     
==========================================
+ Hits         7934     7964      +30     
- Misses      12914    12951      +37     
- Partials      868      870       +2     

Impacted Files	Coverage Δ
pkg/runtime/function/char_length.go	72.72% <72.72%> (ø)
pkg/config/tenant.go	0.00% <0.00%> (ø)
pkg/config/misc.go	0.00% <0.00%> (ø)
pkg/runtime/function/repeat.go	70.37% <0.00%> (ø)
pkg/schema/loader.go	6.74% <0.00%> (+1.12%)	⬆️
pkg/boot/discovery.go	48.62% <0.00%> (+2.16%)	⬆️
pkg/config/file/file.go	73.68% <0.00%> (+3.50%)	⬆️

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[jjeffcaii]

LGTM