Pin which owner writes drop unknown record and manifest fields, that
reads and deletes ignore or erase them, that one undecodable entry fails
a listing, and that v1 wire consumers ignore extra fields.
Move the add-on status subject, delivery phase, status report, staged
manifest and internal add-ons response into contracts/eks/v1 and use it
from the handler, gateway and gatewayfetch, replacing the agent's
duplicate. Wire bytes are unchanged, including the internal add-ons body
that the SDK renderer emits with Go field names; compatibility tests pin
the JSON, the guest printf form and the TSV.
joshsiv-mulga