Context

This PR adds an optional size field to the io.qameta.allure.model.Attachment class to eliminate HEAD HTTP roundtrips for remote attachments (when they are implemented, of course — but here it is a chicken and egg problem).

This change aligns with ongoing efforts to add optional size support across Allure implementations.
See related PR in allure-js: allure-framework/allure-js#1302

Changes:

• Added optional Long size field to Attachment class
• Added getSize() and setSize() methods following existing patterns
• Field is nullable to maintain backward compatibility

Benefits:

• Enables efficient attachment handling when size is already known from JSON
• Eliminates need for reflection-based workarounds
• Maintains backward compatibility with existing code
• Follows Allure's established patterns and conventions
• Supports remote attachment workflows without HTTP roundtrips

Checklist

• Sign Allure CLA
• Provide unit tests