feat: add native server-side copy() for attachment duplication

[cschuerings]

Summary

• Add copy() method to AttachmentsService for server-side attachment duplication across active and draft tables
• Implement storage-level copy for AWS S3, Azure Blob Storage, and GCP
• Include safeguards against entity injection, targetKeys override, and copying infected/failed attachments
• Add demo copyIncident bound action in the incident test FE UI showing the parent-entity copy use case

Closes #279

Details

The new copy() API accepts source and target attachment entities with their keys:

const AttachmentsSrv = await cds.connect.to('attachments')
const newAtt = await AttachmentsSrv.copy(
  sourceAttachments, { ID: sourceId },
  targetAttachments, { up__ID: targetParentId }
)

Supports active-to-active, active-to-draft, and draft-to-draft copies. When targeting a draft table, DraftAdministrativeData_DraftUUID must be provided in targetKeys.

As noted in #279, the primary use case is duplicating attachments when copying a parent entity. The test app demonstrates this with a copyIncident action that creates a new draft incident and clones all its attachments in one operation.

Note on multi-level associations: The copy() API operates on a single attachment composition at a time. For scenarios with nested associations (e.g. Catalog → Books → Attachments, as described in #279), consumers need to iterate over the child entities and call copy() for each.

[schiwekM]

Hi @cschuerings,
thanks for the contribution - will review till end of week. Maybe we can automatically add the action the the Attachments aspect so the attachments table automatically has the Copy button.

BR,
Marten