feat: dev mode support custom asset url host

[hubcarl]

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

Add unit test for follow PR:

• https://github.com/eggjs/egg-view-assets/pull/33
• https://github.com/eggjs/egg-view-assets/pull/36

[codecov[bot]]

Codecov Report

Merging #38 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #38   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          196       198    +2     
=========================================
+ Hits           196       198    +2     

Impacted Files	Coverage Δ
config/config.default.js	100.00% <ø> (ø)
app.js	100.00% <100.00%> (ø)

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[hubcarl]

@popomore need review, then publish

[big-camel]

这。。。唉。。。

[big-camel]

为啥不合并呢，在开发时，想用手机访问开发环境只能硬编码了