Enhancement : FlatFileItemReaderBuilder : raise check exception on build

[frigaux]

Given concrete use case (X as record class)

FlatFileItemReaderBuilder<X>()
...
.targetType(X::class.java)
.fieldSetMapper(myFieldSetMapper)
.build()

Expected Behavior
When both fieldSetMapper and targetType are defined, a "BuildCheckException" should be raised in build method with "you cannot define both FieldSetMapper and targetType"

Current Behavior
FieldSetMapper is ignored and result in misunderstanding at runtime

Best regard