Add compile-time safety to FlatFileItemReaderBuilder DSL

[snowykte0426]

Summary

This PR enhances the FlatFileItemReaderBuilder to prevent mutual
exclusivity violations between targetType() and fieldSetMapper()
methods at compile time rather than runtime, resolving issue #4888.

Changes

• Enhanced DSL with Stage Pattern: Introduced
  FieldSetMapperStage<T> and TargetTypeStage<T> interfaces
• Compile-time Safety: Prevents incompatible method combinations
  through type system

Implementation Details

Before (Runtime Error)

// This would compile but fail at runtime
new FlatFileItemReaderBuilder<Foo>()
    .name("fooReader")
    .resource(resource)
    .lineTokenizer(tokenizer)
    .targetType(Foo.class)
    .fieldSetMapper(mapper)  // IllegalStateException at runtime
    .build();

After (Compile-time Safety)

  // This won't compile - IDE shows error immediately
  new FlatFileItemReaderBuilder<Foo>()
      .name("fooReader")
      .resource(resource)
      .lineTokenizer(tokenizer)
      .targetType(Foo.class)
      .fieldSetMapper(mapper)  // Compile error: method not available
      .build();

Usage Patterns

  // Pattern 1: Using targetType() - returns TargetTypeStage<T>
  new FlatFileItemReaderBuilder<Foo>()
      .name("fooReader")
      .resource(resource)
      .delimited()
      .names("field1", "field2")
      .targetType(Foo.class)
      .beanMapperStrict(true)  // targetType-specific methods available
      .build();

  // Pattern 2: Using fieldSetMapper() - returns FieldSetMapperStage<T>
  new FlatFileItemReaderBuilder<Foo>()
      .name("fooReader")
      .resource(resource)
      .lineTokenizer(tokenizer)
      .fieldSetMapper(fieldSet -> new Foo())
      .saveState(false)  // common methods available
      .build();

Fixes
Issue #4888