object: Provide helper methods for system attributes

cthulhu-rider commented 1 month ago

Is your feature request related to a problem? Please describe.

I'm always frustrated when user needs to do following to set the object expiration:

        attrs := obj.Attributes()
        var expAttrFound bool
        expAttrValue := strconv.FormatUint(exp, 10)

        for i := range attrs {
            if attrs[i].Key() == object.AttributeExpirationEpoch {
                attrs[i].SetValue(expAttrValue)
                expAttrFound = true
                break
            }
        }

        if !expAttrFound {
            index := len(attrs)
            attrs = append(attrs, object.Attribute{})
            attrs[index].SetKey(object.AttributeExpirationEpoch)
            attrs[index].SetValue(expAttrValue)
        }

        obj.SetAttributes(attrs...)

he should care about: expiration is an attribute, it should be unique and formatted correctly