Add compile-time safety to FlatFileItemReaderBuilder DSL

Enhance FlatFileItemReaderBuilder to prevent mutual exclusivity violations
between targetType() and fieldSetMapper() methods at compile time rather
than runtime.

The implementation uses a stage-based DSL pattern where:
- fieldSetMapper() returns FieldSetMapperStage<T> interface
- targetType() returns TargetTypeStage<T> interface
- Each stage only exposes methods compatible with the chosen mapping strategy

This resolves GitHub issue #4888 by providing immediate IDE feedback when
developers attempt to use incompatible method combinations, improving
developer experience and preventing runtime errors.

Signed-off-by: snowykte0426 <[email protected]>

Author: snowykte0426