optimize: support SetCustomFormValueFunc to define custom ctx.FormValue

[li-jin-gou]

What type of PR is this?

optimize

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支持 SetCustomFormValueFunc 用于自定义 ctx.FormValue 函数，内部库提供类似 net/http 的实现。

Which issue(s) this PR fixes:

[codecov]

Codecov Report

Base: 69.65% // Head: 69.64% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (9ed9503) compared to base (fefa4e1).
Patch coverage: 71.79% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #473      +/-   ##
===========================================
- Coverage    69.65%   69.64%   -0.02%     
===========================================
  Files           93       93              
  Lines         8736     8752      +16     
===========================================
+ Hits          6085     6095      +10     
- Misses        2290     2296       +6     
  Partials       361      361              

Impacted Files	Coverage Δ
pkg/app/context.go	84.30% <66.66%> (-0.54%)	⬇️
pkg/route/engine.go	63.19% <100.00%> (+0.49%)	⬆️
pkg/common/timer/timer.go	61.90% <0.00%> (-19.05%)	⬇️
pkg/app/client/client.go	74.68% <0.00%> (+1.26%)	⬆️

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[li-jin-gou]

Conclusion: Use engine.Set*** configuration.