APIv2: Handle native time.Time in serialized structs

[tandr]

Is your feature request related to a problem? Please describe.
Looking at the https://golang.org/cl/187857 I see handling of regular types, but not the time.Time which I used very often. If I understood the intent correctly, it should allow to handle "direct" structs with proto serialization

Describe the solution you'd like
Extend default registration of types with time.Time (and maybe time.Duration)?

[dsnet]

Duplicate of #414