feat: export singleton orm client

[elrrrrrrr]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

• 在 @eggjs/tegg-orm-plugin新增 Orm 类，用于获取当前 leroic realm 实例

• this.orm.client

• this.orm.getClient(name)

• 试用场景 - ref

• Model 添加 dataSource 参数，适配多数据源

Description of change

• 新增单例类 Orm
• 修改 orm ormLoadUnitHook，注册对应 this.orm 实例信息

[elrrrrrrr]

CI 依赖 !83

[codecov]

Codecov Report

Base: 92.54% // Head: 92.73% // Increases project coverage by +0.18% 🎉

Coverage data is based on head (6f2a241) compared to base (b32d9b8).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   92.54%   92.73%   +0.18%     
==========================================
  Files         212      214       +2     
  Lines        4455     4488      +33     
  Branches      454      458       +4     
==========================================
+ Hits         4123     4162      +39     
+ Misses        332      326       -6     

Impacted Files	Coverage Δ
core/orm-decorator/src/decorator/Model.ts	100.00% <100.00%> (ø)
plugin/orm/app.ts	96.96% <100.00%> (+3.63%)	⬆️
plugin/orm/lib/LeoricRegister.ts	100.00% <100.00%> (+4.65%)	⬆️
plugin/orm/lib/ORMLoadUnitHook.ts	100.00% <100.00%> (ø)
plugin/orm/lib/SingletonORM.ts	100.00% <100.00%> (ø)
plugin/orm/lib/DataSourceManager.ts	76.92% <0.00%> (+23.07%)	⬆️

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