[upload] make `assets_root_dir` independent of trailing "/"

At the moment:

• StacUploadTransactionsHandler.publish_item_and_push_assets(..., assets_root_dir="/my/asset/dir") would raise error
• StacUploadTransactionsHandler.publish_item_and_push_assets(..., assets_root_dir="/my/asset/dir/") would work

I suggest using os.path.relpath instead of str.replace to make asset path relative.