log4j-docgen: Support boxed and native Java types in XSD generation (#190)

Previously, `SchemaGenerator` did not handle configuration attributes with boxed types (e.g., `Integer`, `Boolean`), leading to their omission from the generated XSD schema.

This update introduces:

* Support for boxed Java types as configuration attributes.
* Improved handling of other native Java types that map to XML built-in data types (e.g., `BigDecimal`, `URL`).

These enhancements ensure that all relevant configuration attributes are accurately represented in the schema.

Fixes: #135
4 files changed