Add wait API endpoint for waiting on process exit

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

linux/shim/v1/shim.proto

@@ -47,6 +47,8 @@ service Shim {
 	rpc ShimInfo(google.protobuf.Empty) returns (ShimInfoResponse);
 
 	rpc Update(UpdateTaskRequest) returns (google.protobuf.Empty);
+
+	rpc Wait(WaitRequest) returns (WaitResponse);
 }
 
 message CreateTaskRequest {
@@ -152,3 +154,12 @@ message StartResponse {
 	string id = 1;
 	uint32 pid = 2;
 }
+
+message WaitRequest {
+	string id = 1;
+}
+
+message WaitResponse {
+	uint32 exit_status = 1;
+	google.protobuf.Timestamp exited_at = 2 [(gogoproto.stdtime) = true, (gogoproto.nullable) = false];
+}