log4j-docgen: Support attributes as a union of strict type and String

This update enhances the generated XML schema by allowing each attribute to accept either its strict, expected type or a `${...}` expression. This accommodates use cases where property substitution is used, but at the same time allows IDE auto-completions.

> [!WARNING]
> This PR depends on #190 and should not be reviewed until that is merged.

Closes #136
3 files changed