fn: Status should expose if data was served from cache. (#1123)

This is useful in scenarios where gRPC client might want
to reliably observe/report the status latency metrics
and remove any possible duplicates. If the status query
was served from cache, then these latencies show last
execution latency.

api/agent/grpc/runner.proto

@@ -72,6 +72,7 @@ message RunnerStatus {
     string createdAt = 8; // call latency details: initialization time
     string startedAt = 9; // call latency details: start time in container
     string completedAt = 10; // call latency details: end time
+    bool cached = 11; // true if status response was provided from cache
 }
 
 service RunnerProtocol {