Tests: Add client library + service stress test

Add initial version of the client library & service stress test.
This tests tries to push the producer & service to its limits
by spawning a configurable number of processes and threads,
each writing events at a customizable rate.

Some work is still left, I still want to tune the configs and
reason on what to expect. Meanwhile checking in the harness.

Change-Id: Iea0b01e52018125e44f42c5b61c687c60315893b
diff --git a/BUILD.gn b/BUILD.gn
index 0a1b23c..3db5eba 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -81,6 +81,7 @@
   all_targets += [
     ":perfetto_integrationtests",
     "test:client_api_example",
+    "test/stress_test",
   ]
 }