add get_field function to adapter

[woylie]

relates to #238

[codecov]

Codecov Report

Patch coverage: 85.71% and project coverage change: -0.11 ⚠️

Comparison is base (ed29d80) 87.85% compared to head (18fc239) 87.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #383      +/-   ##
==========================================
- Coverage   87.85%   87.75%   -0.11%     
==========================================
  Files          15       15              
  Lines         873      874       +1     
==========================================
  Hits          767      767              
- Misses        106      107       +1     

Impacted Files	Coverage Δ
lib/flop/adapter/ecto.ex	94.66% <80.00%> (-1.05%)	⬇️
lib/flop/schema.ex	97.77% <100.00%> (-0.10%)	⬇️

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