An interface to customize download scheme for other download handlers

[dingld]

A little modification is made on the interface to parse download scheme, scrapy.core.downloader.handler, making it more friendly to add customized download handlers.
Say, when you want to handle the http request via a render engine, without hacking the native download middleware.

Resolves #3354

[codecov]

Codecov Report

Merging #3360 into master will decrease coverage by 0.02%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #3360      +/-   ##
==========================================
- Coverage   84.34%   84.32%   -0.03%     
==========================================
  Files         167      167              
  Lines        9359     9365       +6     
  Branches     1388     1390       +2     
==========================================
+ Hits         7894     7897       +3     
- Misses       1209     1211       +2     
- Partials      256      257       +1

Impacted Files	Coverage Δ
scrapy/core/downloader/handlers/__init__.py	75% <42.85%> (-5%)	⬇️
scrapy/utils/trackref.py	86.48% <0%> (+2.7%)	⬆️