feat(RDV-1664): add PDF and PNG document properties handling

[diogopaulo]

Summary

• Add DocumentProperties structure and wire it through UltimatePDF_ExternalLogic and ScreenshotOptions so PDFs and screenshots can carry title, author, subject and keywords metadata.
• Introduce PDFMetadataUtil and PNGMetadataUtil utilities to embed metadata into generated PDFs and PNG screenshots.
• Add a new UltimatePDF_ExternalLogic.UnitTests project with coverage for the metadata utilities, logger warnings, and metadata performance.

Test Plan

• Run UltimatePDF_ExternalLogic.UnitTests and confirm all tests pass.
• Generate a PDF with document properties set and verify metadata via a PDF reader.
• Generate a screenshot with metadata set and verify PNG tEXt/iTXt chunks contain the expected values.
• Verify behavior when properties are omitted/empty (no regressions in existing PDF/screenshot output).