Add Global ID support to add() in AttachmentManager

[knoopum]

With a layer that contains copies of the features in another layer, but without the attachments, and you want to transfer over the attachments, it would be helpful to be able to rely on matching the global IDs between the layers (assuming the global IDs were preserved while making the copy.) The feature object IDs cannot be relied upon to be consistent between the copy and original, as edits may have created gaps in the original's object ID series, which are eliminated when creating the copy and the object IDs get renumbered automatically.

Currently the add() method of the AttachmentManager requires passing an object ID. While one can use the global ID to find the matching feature's object ID in the copied layer to use with add(), it would be simpler if one could directly pass the global ID.

So the feature request is to add a gid parameter to add(), and require only one of oid or gid.