Add exporter-based schema annotation generation for Maven and Ant

[koentsje]

Summary

• Replaces the self-contained Gradle task with a lean exporter-based version (~230 lines vs 1096) that delegates to SchemaAnnotationExporter via ExporterType.SCHEMA
• Adds Maven mojo (generateSchemaAnnotations) extending AbstractGenerationMojo
• Adds Ant task (<schemaAnnotations/>) extending ExporterTask with registration in HibernateToolTask
• All three build tools share the same exporter infrastructure: SchemaAnnotationExporter, SchemaAnnotationHelper, and SchemaAnnotation.ftl

Test plan

• Gradle functional test passes (existing from PR)
• Maven mojo unit tests (3 tests: generate, with package, without package)
• Ant unit tests (getName, setSchemaPackage)
• Ant integration tests (3 tests: generate with FK verification, with package, without package)
• Full clean build passes