Determine how to deal with public log lengths

[nventuro]

Public logs currently always have the full length of 13 fields, with non-emitted fields being padded with zeroes. We need to know the actual length in order to not publish unnecessary data to DA (and should not rely on simply trimming back the zeroes, which is what we're temporarly doing as of #11641) and to properly handle the payload when processing logs in contracts.