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/async_sender.cpp

@@ -17,7 +17,7 @@ struct shared_test_data {
 	error_code fail = asio::error::not_connected;
 
 	const std::string connect = encoders::encode_connect(
-		"", std::nullopt, std::nullopt, 10, false, {}, std::nullopt
+		"", std::nullopt, std::nullopt, 60, false, {}, std::nullopt
 	);
 	const std::string connack = encoders::encode_connack(
 		false, reason_codes::success.value(), {}