Allow to pass keyDecodingStrategy in first(decoding:) and all(decoding:)

[valerianb]

When inserting, you can specify a keyEncodingStrategy:

try db.insert(into: myTable)
  .model(myModel, keyEncodingStrategy: .convertToSnakeCase)
  .run()

This change allows to pass a keyDecodingStrategy when using the convenience methods to decode models directly:

db.select()
  .from(myTable)
  .all(decoding: MyModel.self, keyDecodingStrategy: .convertFromSnakeCase)

[codecov-commenter]

Codecov Report

Merging #161 (baf26a3) into main (a5c3df4) will not change coverage.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #161   +/-   ##
=======================================
  Coverage   69.27%   69.27%           
=======================================
  Files          94       94           
  Lines        3486     3486           
=======================================
  Hits         2415     2415           
  Misses       1071     1071           

Flag	Coverage Δ
unittests	69.27% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Sources/SQLKit/Builders/SQLQueryFetcher.swift	28.94% <0.00%> (ø)

[gwynne]

This was addressed by #172