Adding frequency to SubscribeAttributes (#81)

* Adding frequency to SubscribeAttributes

Frequency will be used to throttle published events that are to be sent
between devices. This is commonly used when cloud/mobile
applications wish to subscribe to high frequency vehicle topics

#80

* use max_frequency in lieu of frequency

* Change frequency to sample period in ms pre reviewers feedback

* Fix dumb error

uprotocol/core/usubscription/v3/usubscription.proto

@@ -118,6 +118,16 @@ message SubscribeAttributes {
   // Any additional producer specific information that the consumer must send
   // inside of a subscription request
   repeated google.protobuf.Any details = 2;
+
+  // Desired Sampling Period (measured in milliseconds) the subscriber wishes to receive events 
+  // for remote only topics. Device dispatchers (i.e. streamers) use this
+  // attribute to reduce the publication rates of events sent between devices.
+  // This attribute is commonly used for mobile/cloud subscribing to vehicle topics
+  // that are published at a high rate.<br>
+  // If the desired sampling period set by the subscriber is lower than the original publisher's
+  // publication period, the attribute is ignored.<br>
+  // **__NOTE:__** If this attribute is missing, the sampling period is set by the publisher.
+  optional uint32 sample_period_ms = 3;
 }