feat(sharding): support sum function

[wbtlb]

What is changed and how it works?

What's Changed:

• support sum function

[codecov]

Codecov Report

Merging #388 (eae09b7) into master (df61eb4) will increase coverage by 6.38%.
The diff coverage is 82.17%.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
+ Coverage   66.97%   73.36%   +6.38%     
==========================================
  Files           7        6       -1     
  Lines         427      732     +305     
==========================================
+ Hits          286      537     +251     
- Misses        133      184      +51     
- Partials        8       11       +3     

Flag	Coverage Δ
unittests	73.36% <82.17%> (+6.38%)	⬆️

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

Impacted Files	Coverage Δ
pisa-controller/pkg/proxy/server.go	20.00% <0.00%> (-2.23%)	⬇️
pisa-controller/pkg/webhook/server.go	26.66% <0.00%> (ø)
pisa-controller/pkg/proxy/http.go	51.35% <44.11%> (-6.55%)	⬇️
pisa-controller/pkg/webhook/injection.go	59.02% <82.56%> (+20.08%)	⬆️
pisa-controller/pkg/proxy/builder.go	90.97% <93.25%> (-0.25%)	⬇️

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[xuanyuan300]

Good job.