GetCapacityResponse: add maximum_volume_size

This new field has a more precisely defined semantic than the existing
available_capacity. It is intended for checking in advance in which
topology a CreateVolume call might succeed. Checking that based
on available_capacity is less precise, for example because
fragmentation might prevent creating a single volume that uses
all available capacity.

The new value has to be returned in a message to allow the caller to
determine whether the value was provided.

Author: pohly

csi.proto

@@ -947,6 +947,26 @@ message GetCapacityResponse {
   // storage. This field is REQUIRED.
   // The value of this field MUST NOT be negative.
   int64 available_capacity = 1;
+
+  // The largest size that could have been used in a CreateVolume call
+  // to create a volume with the same parameters as those in
+  // GetCapacityRequest.
+  //
+  // If `volume_capabilities` or `parameters` is
+  // specified in the request, the Plugin SHALL take those into
+  // consideration when calculating the maximum volume size of the
+  // storage.
+  //
+  // This field is OPTIONAL. If a Plugin has no restrictions for the
+  // size of volumes (for example, linear storage with no fragmentation
+  // issues and no technical limitation for individual volumes), then
+  // can leave this field unset and only return available_capacity.
+  Capacity maximum_volume_size = 2 [(alpha_field) = true];
+}
+
+message Capacity {
+  // Number of bytes. MUST NOT be negative.
+  int64 value = 1;
 }
 message ControllerGetCapabilitiesRequest {
   // Intentionally empty.