Add new fields in asset struct

[overag3]

New fields appear when getting assets:

• contentType
• lastModified
• lastDownload
• uploader
• uploaderIP
• fileSize
• blobCreated

Changes TODO in this struct:

gonexus/rm/assets.go

Line 20 in eab4eb0

type RepositoryItemAsset struct {

[overag3]

New checksum fields are available on specific assets:

• sha256
• sha512