[WIP] feat: impl the real ContextModel

[fengmk2]

see https://github.com/eggjs/egg/issues/1885

Checklist

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

Affected core subsystem(s)

Description of change

[codecov-io]

Codecov Report

Merging #36 into master will decrease coverage by 6.12%. The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage     100%   93.87%   -6.13%     
==========================================
  Files           5        5              
  Lines          37       49      +12     
==========================================
+ Hits           37       46       +9     
- Misses          0        3       +3

Impacted Files	Coverage Δ
agent.js	100% <ø> (ø)	:arrow_up:
app/extend/context.js	100% <100%> (ø)	:arrow_up:
lib/loader.js	92.1% <66.66%> (-7.9%)	:arrow_down:

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