Which issue does this PR close?

• Closes bug(udf): scalar UDFs over nested Arrow types cannot be registered #58 .

Rationale for this change

ScalarFunction.argTypes() returned List<ArrowType> and returnType() returned ArrowType. In Java Arrow, ArrowType is a leaf marker for the type kind: it is self-describing for primitives like Int32 or Float64, but for nested types (List, Struct, Map, FixedSizeList) the element / member / key / value types live on the parent Field's children list, not inside ArrowType. ArrowType.List is literally a no-field marker class.

A Java UDF author therefore had no way to declare a typed nested signature. Trying argTypes() = List.of(new ArrowType.List()) blew up at registration time:

IllegalArgumentException: Lists have one child Field. Found: none
  at SessionContext.serializeSchemaIpc(SessionContext.java:398)
  at SessionContext.registerUdf(SessionContext.java:391)

This blocked the entire family of nested-type UDFs that exist as built-ins in DataFusion's datafusion-functions-nested crate (array_length, cardinality, array_has, array_position, flatten, map_keys, map_values, arrows_zip, ...). Anyone porting Spark UDFs over ArrayType / StructType / MapType columns to DataFusion-Java hit this on the first attempt.

The Rust API does not have this problem because DataType::List(Arc<Field>) carries the child field inline. Switching the Java interface from ArrowType to Field is the structural mirror: Field is the only type that can carry children, so it's the type the interface has always needed to use.

What changes are included in this PR?

See commit log

Are these changes tested?

Yes -- 5 new tests over and above the existing 12.

Are there any user-facing changes?

Yes -- a source-breaking signature change to the public ScalarFunction interface. Existing primitive UDFs become slightly more verbose:

// Before:
public List<ArrowType> argTypes() { return List.of(INT32); }
public ArrowType returnType() { return INT32; }

// After:
public List<Field> argFields() { return List.of(Field.nullable("arg0", INT32)); }
public Field returnField() { return Field.nullable("return", INT32); }

Nested-type UDFs that were previously impossible to declare can now be expressed by attaching child fields. For example, a UDF over List<Int32> returning the list length:

// List<Int32> argument: the element type lives in the Field's children list.
public List<Field> argFields() {
  return List.of(
      new Field(
          "vals",
          FieldType.nullable(new ArrowType.List()),
          List.of(Field.nullable("item", INT32))));
}

public Field returnField() {
  return Field.nullable("len", INT32);
}

Same shape works for Struct<a: Int32, b: Int32> (children are the member fields) and Map<Utf8, Int32> (children describe the key/value entries struct).

The repo is pre-release, which makes this the right time to tighten the interface before downstream callers accumulate.