gogoproto.customtype is used to have go-digest.Digest instead of string.
While it is convinient, protoc-gen-go doesn't support the extension
and that blocks #6564.
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
This change allows implementations to resolve the location of the actual data
using OCI descriptor fields such as MediaType.
No OCI descriptor field is written to the store.
No change on gRPC API.
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
