Allow per item validators to be specified by string

[rvandam]

We ran into a problem where another extension required our settings to be json serializable which was barfing on the class names used for specifying json schemas. I'm separately working on a fix for that extension but since under the hood the classes are just getting converted to obj.__name__ anyway I wondered why I couldn't just specify them that way to begin with.

[rvandam]

This latest version should pass all tests, including the new one, at least on 3.9-3.11 (I don't have 3.8 installed to test but it should be fine).

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6edb037) 79.39% compared to head (a75e6fb) 79.39%.

:exclamation: Current head a75e6fb differs from pull request most recent head 5acd876. Consider uploading reports for the commit 5acd876 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #419   +/-   ##
=======================================
  Coverage   79.39%   79.39%           
=======================================
  Files          76       76           
  Lines        3222     3222           
  Branches      534      534           
=======================================
  Hits         2558     2558           
  Misses        593      593           
  Partials       71       71           

Files Changed	Coverage Δ
spidermon/contrib/scrapy/pipelines.py	97.80% <100.00%> (ø)

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