Support Datastore entity keys

[nevillelyh]

Possibly through a field annotation.
https://cloud.google.com/datastore/docs/concepts/entities
https://github.com/googleapis/googleapis/blob/master/google/datastore/v1beta3/entity.proto

For something like Person(@key email: String, name: String, age: Int)

• partition_id
  • project_id: "" or from class level @projectId("my-project") annotation
  • namespace_id "", case class namespace, or from class level @namespace("com.myorg") annotation
• path (repeated but we can allow only 1 level for simplicity?)
  • kind: case class name or from class level @kind annotation
  • id/name: Int/String field value