Proposing `Quantity`

[Meng Zhu]

Hi:

We are proposing to add a primitive type `Quantity` to facilitate several
ongoing projects. The schema will be:

message Quantity {
  required string name;
  optional value.scalar scalar;
}

You can find more details such as motivation, current, and future use cases
in this design doc
<https://docs.google.com/document/d/1WbRKmqsos1-IBJ9VjpT4kNIp2Peyvx2k4UphRaFVYFU/edit?usp=sharing>.
Feel free leave comments and questions.

Thanks,
Meng