fix: add key any to Context

[whxaxes]

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

#2644

因为最近 koa 发的 typings 版本，在 Context 中将 [key: string]: any 去掉了，所以在我们这里补上。

在想 EggApplication 是否也需要加上 key any 参数，来保证至少不会抛编译错误？

[codecov]

Codecov Report

Merging #2650 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2650   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files          29       29           
  Lines         822      822           
=======================================
  Hits          819      819           
  Misses          3        3

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 65a43aa...f6fee7f. Read the comment docs.