Add WebSocket support for exchanging events with external clients.

This exposes Broker's new WebSocket support in Zeek. To enable it,
call `Broker::listen_websocket()`. Zeek will then start listening on
port 9997 for incoming WebSocket connections.

See the Broker documentation for a description of the message format
expected over these WebSocket connections.

testing/btest/Baseline/broker.web-socket-events/client.output

@@ -0,0 +1,4 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+ping ['my-message', 1]
+ping ['my-message', 2]
+ping ['my-message', 3]

testing/btest/Baseline/broker.web-socket-events/server.output

@@ -0,0 +1,6 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+sender added peer: endpoint=127.0.0.1 msg=handshake successful
+sender got pong: my-message, 1
+sender got pong: my-message, 2
+sender got pong: my-message, 3
+sender lost peer: endpoint=127.0.0.1 msg=lost connection to client

testing/btest/broker/web-socket-events.zeek

@@ -0,0 +1,124 @@
+# @TEST-GROUP: broker
+#
+# @TEST-PORT: BROKER_PORT
+#
+# @TEST-EXEC: btest-bg-run server "zeek -b %INPUT >output"
+# @TEST-EXEC: btest-bg-run client "python3 ../client.py >output"
+#
+# @TEST-EXEC: btest-bg-wait 45
+# @TEST-EXEC: btest-diff client/output
+# @TEST-EXEC: btest-diff server/output
+
+redef exit_only_after_terminate = T;
+redef Broker::disable_ssl = T;
+
+global event_count = 0;
+
+global ping: event(msg: string, c: count);
+
+event zeek_init()
+    {
+    Broker::subscribe("/zeek/event/my_topic");
+    Broker::listen_websocket("127.0.0.1", to_port(getenv("BROKER_PORT")));
+    }
+
+function send_event()
+    {
+    ++event_count;
+    local e = Broker::make_event(ping, "my-message", event_count);
+    Broker::publish("/zeek/event/my_topic", e);
+    }
+
+event Broker::peer_added(endpoint: Broker::EndpointInfo, msg: string)
+    {
+    print fmt("sender added peer: endpoint=%s msg=%s", endpoint$network$address, msg);
+    send_event();
+    }
+
+event Broker::peer_lost(endpoint: Broker::EndpointInfo, msg: string)
+    {
+    print fmt("sender lost peer: endpoint=%s msg=%s", endpoint$network$address, msg);
+    terminate();
+    }
+
+event pong(msg: string, n: count)
+    {
+    print fmt("sender got pong: %s, %s", msg, n);
+    send_event();
+    }
+
+
+@TEST-START-FILE client.py
+import asyncio, websockets, os, time, json, sys
+
+ws_port = os.environ['BROKER_PORT'].split('/')[0]
+ws_url = 'ws://localhost:%s/v1/messages/json' % ws_port
+topic = '"/zeek/event/my_topic"'
+
+def broker_value(type, val):
+    return {
+            '@data-type': type,
+            'data': val
+            }
+
+async def do_run():
+    # Try up to 30 times.
+    connected  = False
+    for i in range(30):
+        try:
+            ws = await websockets.connect(ws_url)
+            connected  = True
+
+            # send filter and wait for ack
+            await ws.send('[%s]' % topic)
+            ack_json = await ws.recv()
+            ack = json.loads(ack_json)
+            if not 'type' in ack or ack['type'] != 'ack':
+                print('*** unexpected ACK from server:')
+                print(ack_json)
+                sys.exit()
+        except Exception as e:
+            if not connected:
+                print('failed to connect to %s, try again (%s)' % (ws_url, e), file=sys.stderr)
+                time.sleep(1)
+                continue
+            else:
+                print('exception: %s' % e, file=sys.stderr)
+                sys.exit()
+
+        for round in range(3):
+            # wait for ping
+            msg = await ws.recv()
+            msg = json.loads(msg)
+            if not 'type' in msg or msg['type'] != 'data-message':
+                continue
+
+            ping = msg['data'][2]['data']
+            name = ping[0]['data']
+            args = [x['data'] for x in ping[1]['data']]
+            print(name, args)
+
+            # send pong
+            pong = [broker_value('string', 'pong'),
+                    broker_value('vector', [
+                    broker_value('string', args[0]),
+                    broker_value('count', args[1])
+                    ])]
+
+            ev = [broker_value('count', 1), broker_value('count', 1), broker_value('vector', pong)]
+            msg = {
+                'type': 'data-message',
+                'topic': '/zeek/event/my_topic',
+                '@data-type': 'vector', 'data': ev
+            }
+
+            msg = json.dumps(msg)
+            await ws.send(msg)
+
+        await ws.close()
+        sys.exit()
+
+loop = asyncio.get_event_loop()
+loop.run_until_complete(do_run())
+
+@TEST-END-FILE