Update etcd API protobuf files

[jumaffre]

The etcd API definition protobuf file (currently https://github.com/microsoft/ccf-kvs/blob/main/proto/etcd.proto) is a stripped down version of https://github.com/etcd-io/etcd/tree/main/api so that we could port a sample app easily to CCF.

However, we should simply include etcd/api under 3rdparty/etcd/api so that it's easily updatable in the future. I've tried doing this but unfortunately this requires extra dependencies (e.g. gogoproto/gogo.proto) and isn't as trivial to setup.