Make lazy loading Download Handlers optional

[hcoura]

Download Handlers load only when first request for a scheme is needed, this was put in place because s3 download handler used to be very slow to create.

This makes it optional for Download Handlers to be lazy loaded which is relevant when writing custom handlers that have required settings and wants to raise NotConfiguredException during instantiation.

[codecov]

Codecov Report

Merging #3394 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3394      +/-   ##
==========================================
+ Coverage   84.46%   84.49%   +0.02%     
==========================================
  Files         167      167              
  Lines        9362     9372      +10     
  Branches     1390     1391       +1     
==========================================
+ Hits         7908     7919      +11     
  Misses       1199     1199              
+ Partials      255      254       -1

Impacted Files	Coverage Δ
scrapy/core/downloader/handlers/http10.py	100% <100%> (ø)	⬆️
scrapy/core/downloader/handlers/file.py	100% <100%> (ø)	⬆️
scrapy/core/downloader/handlers/datauri.py	93.75% <100%> (+0.41%)	⬆️
scrapy/core/downloader/handlers/__init__.py	83.63% <100%> (+3.63%)	⬆️
scrapy/core/downloader/handlers/ftp.py	98.24% <100%> (+0.03%)	⬆️
scrapy/core/downloader/handlers/http11.py	92.53% <100%> (+0.02%)	⬆️

[kmike]

Looks good, thanks @hcoura! //cc @dangra