allow additional fields to be added to the Pydantic model's schema

[dantownsend]

Related to piccolo-orm/piccolo_admin#73

[codecov-commenter]

Codecov Report

Merging #313 (4623b52) into master (24a3191) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #313   +/-   ##
=======================================
  Coverage   90.56%   90.56%           
=======================================
  Files         125      125           
  Lines        6262     6262           
=======================================
  Hits         5671     5671           
  Misses        591      591           

Impacted Files	Coverage Δ
piccolo/utils/pydantic.py	98.61% <100.00%> (ø)

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