Factor out `ServeProto::Serialiser<UnkeyedValidPathInfo>` and test (cherry picked from commit 139982997eec493a0f74105c427953f6be77da6d) Change-Id: I28e4ba5a681a90d81915a56e6dbaa5456d64f96d
I28e4ba5a681a90d81915a56e6dbaa5456d64f96d