fix & docs & test: 修复全局数据库配置无法生效的问题以及新增测试用例，修改文档说明，新增reader/writer注册函… …数测试用例

[Breeze0806]

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[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.06 ⚠️

Comparison is base (9e41ed7) 92.39% compared to head (9209639) 92.34%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
- Coverage   92.39%   92.34%   -0.06%     
==========================================
  Files         121      123       +2     
  Lines        6591     6635      +44     
==========================================
+ Hits         6090     6127      +37     
- Misses        399      404       +5     
- Partials      102      104       +2     

Impacted Files	Coverage Δ
datax/core/job/container.go	92.04% <100.00%> (+0.15%)	⬆️
datax/plugin/writer/dbms/config.go	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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