feat(managed): Add logic so retain can be used both on vec and smallvec

[tobias-wilfert]

While working on #5747 I would ideally have something like this irregardles of if attachments are a vec or a smallvec (items):

pub fn validate(attachments: &mut Managed<SerializedAttachments>) {
    attachments.retain(
        |attachments| &mut attachments.attachments,
        |attachment, _| utils::attachments::validate(attachment),
    );
}