feat: support ssr for egg framework

[justquanyin]

增加北斗同构插件，用以支持Egg框架下的轻量级同构功能

Checklist

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

Description of change

• 开启 egg-beidou 插件，可使用 ctx.ssr(path,props) 进行同构页面渲染
• path路径支持 全路径、绝对路径，可配置默认文件扩展名
• 支持require.cache缓存是否使用配置
• 支持 render stream
• 支持渲染错误异常处理

[codecov-io]

Codecov Report

Merging #168 into master will decrease coverage by 0.25%.
The diff coverage is 84.09%.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
- Coverage   93.02%   92.77%   -0.26%     
==========================================
  Files          86       87       +1     
  Lines        1577     1605      +28     
==========================================
+ Hits         1467     1489      +22     
- Misses        110      116       +6

Impacted Files	Coverage Δ
packages/egg-beidou/config/config.default.js	100% <100%> (ø)	⬆️
packages/egg-beidou/app/view-middlewares/script.js	100% <100%> (ø)	⬆️
packages/egg-beidou/app/view-middlewares/custom.js	100% <100%> (ø)	⬆️
packages/egg-beidou/app/view-middlewares/render.js	100% <100%> (ø)	⬆️
packages/egg-beidou/app.js	100% <100%> (ø)	⬆️
packages/egg-beidou/app/view-middlewares/style.js	100% <100%> (ø)	⬆️
packages/egg-beidou/lib/beidou.js	78.26% <42.85%> (-21.74%)	⬇️
packages/egg-beidou/app/extend/context.js	90.32% <90.32%> (ø)
packages/beidou-view-rax/lib/rax-view.js	94.11% <0%> (+5.88%)	⬆️
... and 1 more

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