feat(server): allow async bypass function for proxying

[gromo]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

Yes

Motivation / Use-Case

Currently result of bypass function is used immediately and there is no way to make it asynchronous. This update allows returning Promise from bypass function in proxy.

It can be useful for downloading content from original source and "patching" it, instead of storing the whole response.

Breaking Changes

No

Additional Info

[codecov]

Codecov Report

Merging #2696 into v4 will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##               v4    #2696   +/-   ##
=======================================
  Coverage   92.85%   92.85%           
=======================================
  Files          37       37           
  Lines        1316     1316           
  Branches      354      354           
=======================================
  Hits         1222     1222           
  Misses         89       89           
  Partials        5        5           

Impacted Files	Coverage Δ
lib/Server.js	95.98% <100.00%> (ø)

---

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 2bbd36f...ca7c749. Read the comment docs.

[alexander-akait]

/cc @hiroppy @Loonride

[hiroppy]

Thanks