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)