Support the upcoming proxy API of Zyte API

[Gallaecio]

To do:

• [x] Determine where in docs.zyte.com the proxy API reference documentation will be, and update the 2 corresponding references in the docs here to point there.

[codecov[bot]]

Codecov Report

Merging #108 (1e59a18) into master (54206a5) will not change coverage. Report is 2 commits behind head on master. The diff coverage is 100.00%.

:exclamation: Current head 1e59a18 differs from pull request most recent head e4a7e12. Consider uploading reports for the commit e4a7e12 to get more accurate results

@@            Coverage Diff            @@
##            master      #108   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          242       284   +42     
=========================================
+ Hits           242       284   +42     

Files	Coverage Δ
scrapy_zyte_smartproxy/middleware.py	100.00% <100.00%> (ø)

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[akshayphilar]

@Gallaecio great work. Could we go ahead and merge this.