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 b/BUILD
index b3c511a..576dc0e 100644
--- a/BUILD
+++ b/BUILD
@@ -1837,6 +1837,7 @@
     srcs = [
         "protos/perfetto/config/chrome/chrome_config.proto",
         "protos/perfetto/config/data_source_config.proto",
+        "protos/perfetto/config/stress_test_config.proto",
         "protos/perfetto/config/test_config.proto",
         "protos/perfetto/config/trace_config.proto",
     ],