Data model proto definitions

[christopher-arista]

While we have the proto for the service itself, it would be helpful to get the protos for some (or at least one) of the data models that are intended to be used. This will allow us to start prototyping an implementation.

Also, is the intent to publish the data model proto definitions in this repo, similar to https://github.com/openconfig/hercules/tree/master/proto/openconfig?

[robshakir]

Please see #6 -- this is just a first proposal, but perhaps something we can discuss further.