Port PR from PBS-Java: Nexx360: Preserve custom imp.ext fields and add vendor id

[cross-repo-issue-creation]

A PR was merged over on PBS-Java

• Nexx360: Preserve custom imp.ext fields and add vendor id prebid-server-java#4359
• timestamp: 2026-02-04T13:29:23Z

[osulzhenko]

Already in Go:
#4604