feat(clbot): Create Gerrit watcher and basic clbot binary.

gerrit.Watcher is a class which watches the Gerrit stream-events SSH
connection and produces events.

There's a basic CLBot binary as well, to demonstrate driving it to
produce messages on the logging output. It doesn't really do anything
else.

Change-Id: I274fe0a77c8329f79456425405e2fbdc3ca2edf0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/245
Reviewed-by: tazjin <mail@tazj.in>

third_party/gopkgs/github.com/cenkalti/backoff/default.nix

@@ -0,0 +1,12 @@
+{ depot, ... }:
+
+depot.buildGo.external {
+  path = "github.com/cenkalti/backoff/v4";
+
+  src = depot.third_party.fetchFromGitHub {
+    owner = "cenkalti";
+    repo = "backoff";
+    rev = "18fe4ce5a8550e0d0919b680ad3c080a5455bddf";
+    sha256 = "083617p066p77ik0js8wwgb5qzabgvl8wqpkjb8s9alpyqsq2mpg";
+  };
+}