Prototype for `query` endpoint which allows batch querying

[dantownsend]

Related to https://github.com/piccolo-orm/piccolo_api/issues/239

[codecov-commenter]

Codecov Report

Merging #240 (e7797d2) into master (7303065) will decrease coverage by 0.56%. The diff coverage is 64.86%.

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@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
- Coverage   92.52%   91.97%   -0.56%     
==========================================
  Files          33       33              
  Lines        2021     2044      +23     
==========================================
+ Hits         1870     1880      +10     
- Misses        151      164      +13     

Impacted Files	Coverage Δ
piccolo_api/crud/endpoints.py	92.53% <57.14%> (-2.22%)	:arrow_down:
piccolo_api/fastapi/endpoints.py	98.43% <75.00%> (-0.76%)	:arrow_down:
piccolo_api/crud/exceptions.py	85.29% <100.00%> (+1.96%)	:arrow_up:
piccolo_api/crud/validators.py	90.56% <100.00%> (+0.18%)	:arrow_up: