provider: add serp

[Gallaecio]

Subset of #218 that is a first step towards allowing the serp spider from https://github.com/zytedata/zyte-spider-templates to use Serp page objects.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (bf58d54) to head (4aa88b4).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #242   +/-   ##
=======================================
  Coverage   97.75%   97.75%           
=======================================
  Files          14       14           
  Lines        1647     1647           
  Branches      301      301           
=======================================
  Hits         1610     1610           
  Misses         17       17           
  Partials       20       20           

Files with missing lines	Coverage Δ
scrapy_zyte_api/providers.py	94.73% <ø> (ø)

[wRAR]

We need to add the same thing 5 places, I wonder if it can be made more error-prone :)