Consider introducing a ScalarResourceQuantity protobuf message.

As part of introducing quota limits, we're adding a new master::Call for updating quota. This call can take a simplified message that expresses scalar resource quantities:

message ScalarResourceQuantity {
  required string name;
  required Value::Scalar quantity;
}

This greatly simplified the validation code, as well as the UX of the API when it comes to knowing what kind of data to provide.

Ideally, the new quota paths can use this message in lieu of Resource objects, but we'll have to explore backwards compatibility (e.g. registry data).