feat: implement batch map

pkg/apis/proto/map/v1/map.proto

@@ -13,6 +13,14 @@ service Map {
 
   // IsReady is the heartbeat endpoint for gRPC.
   rpc IsReady(google.protobuf.Empty) returns (ReadyResponse);
+
+  // MapStreamFn is a bi-directional streaming rpc which applies a
+  // Map function on each element of the stream and then returns streams
+  // back MapResponse elements.
+  // TODO(map-batch): in the target state when we move the current
+  // unary implementation to bi-di as well, we can rename this and
+  // use a single rpc for both.
+  rpc MapStreamFn(stream MapRequest) returns (stream MapResponse);
 }
 
 /**
@@ -24,18 +32,25 @@ message MapRequest {
   google.protobuf.Timestamp event_time = 3;
   google.protobuf.Timestamp watermark = 4;
   map<string, string> headers = 5;
+  string id = 6;
 }
 
 /**
  * MapResponse represents a response element.
  */
+// TODO(map-batch) - currently this is used by both batch map and unary map.
+// Do we want to have a separate response struct for batch map responses
+// which have only one element instead of a list of responses.
+// In that case we need a different mechanism to indicate that all the responses for a given request
+// have been completed.
 message MapResponse {
   message Result {
     repeated string keys = 1;
     bytes value = 2;
     repeated string tags = 3;
   }
   repeated Result results = 1;
+  string id = 2;
 }
 
 /**