feat(allure-model): add optional size field to Attachment class

[noomorph]

Description

This PR adds an optional size field to the io.qameta.allure.model.Attachment class to support 'no-fetch' flows for attachments.

Context

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

Related Issues

Fixes the need for AttachmentMixIn workarounds and reflection-based size access.