Add ProGuardTransformer to merge META-INF/proguard/*.pro files

[Copilot]

Fixes #1696

Summary

Adds a new ProGuardTransformer resource transformer that merges ProGuard rules files under META-INF/proguard/** by concatenating their lines — similar to how ServiceFileTransformer works for META-INF/services files.

Changes

• ProGuardTransformer.kt: New transformer extending PatternFilterableResourceTransformer. Matches META-INF/proguard/** by default; accumulates lines per file path; writes merged output to the shadow JAR.
• ShadowJar.kt: Added mergeProGuardFiles(action) convenience method (mirrors mergeServiceFiles).
• Unit test (src/test/.../ProGuardTransformerTest.kt): Tests canTransformResource, multi-file merging, modifyOutputStream output, and alternate path configuration.
• Functional test (src/functionalTest/.../ProGuardTransformerTest.kt): End-to-end test verifying both the short mergeProGuardFiles() syntax and the generic transform<ProGuardTransformer> syntax.