Add optional 'Unavailability' to resource offers to provide maintenance awareness.

message Interval {
  optional double start = 1; // Time, in seconds since the Epoch.
  optional double duration = 2; // Time, in seconds.
}

message Offer {
  // Existing fields
  ...
 
  // Signifies that the resources in this Offer are part of a planned
  // maintenance schedule in the specified window.  Any tasks launched
  // using these resources may be killed when the window arrives.
  // This field gives additional information about the maintenance.
  // The maintenance may not necessarily start at exactly at this interval,
  // nor last for exactly the duration of this interval.
  optional Interval unavailability = 9;
}