Add ZYTE_API_ENABLED

[Gallaecio]

Resolves #12

[codecov]

Codecov Report

Merging #40 (a4706f0) into main (e06219d) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
+ Coverage   99.31%   99.34%   +0.03%     
==========================================
  Files           3        3              
  Lines         145      152       +7     
==========================================
+ Hits          144      151       +7     
  Misses          1        1              

Impacted Files	Coverage Δ
scrapy_zyte_api/handler.py	99.02% <100.00%> (+0.07%)	⬆️

[kmike]

I think it might be better to have a separate option for enabling all requests to go through Zyte API, because of the following reasons:

1. Usually ..._ENABLED options just turn on/off the middleware.
2. It may also be tricky to set this setting to None from the command line, if I recall correctly.
3. We shoud consider future Zyte API features, with data extraction use cases: if user only wants the extraction features, they'd need to set ZYTE_API_ENABLED to None, which is a bit strange.
4. Instead of relying on the user to specify correct request parameters to use Zyte API as a downloader, it'd be nice to set them on our side. This may involve not only passing static default parameters, but also taking request method, body and headers in account.

[Gallaecio]

3 is not fully clear to me, but I don’t think it matters at this point.

Shall we deal with 4 on a follow-up pull request, or here? I would love to work on it, but I also would like to discuss the API a bit before working on an implementation.

[Gallaecio]

Feel free to suggest alternatives to ZYTE_API_ALL (ZYTE_API_ALL_REQUESTS? ZYTE_API_FORCE? ZYTE_API_USE_BY_DEFAULT?)

[BurnzZ]

LGTM @Gallaecio !

[Gallaecio]

Closing in favor of #41