Add function to set keep alive/ping interval

Summary:
related to T13566
- mqtt_client has a new keep_alive(seconds) function
- keep_alive(0) disables ping
- if keep_alive() is not called, the client assumes keep_alive=10
- the client respects server_keep_alive if sent by the broker

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D27557

test/integration/executors.cpp

@@ -26,7 +26,7 @@ BOOST_AUTO_TEST_CASE(async_run) {
 
 	// packets
 	auto connect = encoders::encode_connect(
-		"", std::nullopt, std::nullopt, 10, false, {}, std::nullopt
+		"", std::nullopt, std::nullopt, 60, false, {}, std::nullopt
 	);
 	auto connack = encoders::encode_connack(
 		false, reason_codes::success.value(), {}