public <R> R getValue(T structInstance, Class<R> expectedType) throws IOException {
		return ReflectionHelper.getFieldValue(structInstance, field, expectedType);
	}
Creates a FieldMappingInfo instance, used when the structure is variable length and there is
no pre-defined Ghidra Structure data type.

@param <T> structure mapped class type
@param field java field
@param fieldName name of Ghidra structure field
@param signedness {@link Signedness} enum
@param length override of structure field, or -1
@return new {@link FieldMappingInfo} instance
Ghidra/Features/Base/src/main/java/ghidra/app/util/bin/format/golang/structmapping/FieldMappingInfo.java