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>

fun/clbot/gerrit/default.nix

@@ -0,0 +1,18 @@
+{ depot, ... }:
+
+let
+  inherit (depot.fun) clbot;
+  inherit (depot.third_party) gopkgs;
+in
+depot.nix.buildGo.package {
+  name = "code.tvl.fyi/fun/clbot/gerrit";
+  srcs = [
+    ./watcher.go
+  ];
+  deps = [
+    clbot.gerrit.gerritevents
+    gopkgs."github.com".cenkalti.backoff.gopkg
+    gopkgs."github.com".golang.glog.gopkg
+    gopkgs."golang.org".x.crypto.ssh.gopkg
+  ];
+}