Examples rework

Summary:
related to T15261, T15262
- all examples are added to CI checks
	- as a result, CMakeLists.txt in example folder is reworked to build all examples instead of one
	- also removed chapter Building with CMake temporarily (it will be back)
- all examples accept runtime arguments (brokers, port, clientid) (default: hivemq public broker)
- implemented example suggestions by R.Perez, most notable:
	- all examples include mqtt headers they use (no more unified headers)
	- multithreaded examples use thread_pool instead of asio::io_context
- all examples use logger with log_level::info by default

Reviewers: ivica

Reviewed By: ivica

Subscribers: iljazovic, miljen

Differential Revision: https://repo.mireo.local/D32602

example/hello_world_over_tls.cpp

@@ -7,21 +7,30 @@
 
 //[hello_world_over_tls
 #include <iostream>
+#include <string>
 
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/detached.hpp>
 #include <boost/asio/ssl.hpp>
 #include <boost/asio/ip/tcp.hpp>
 
-#include <async_mqtt5.hpp>
+#include <async_mqtt5/logger.hpp>
+#include <async_mqtt5/mqtt_client.hpp>
+#include <async_mqtt5/types.hpp>
+
+struct config {
+	std::string brokers = "broker.hivemq.com";
+	uint16_t port = 8883; // 8883 is the default TLS MQTT port.
+	std::string client_id = "async_mqtt5_tester";
+};
 
 // External customization point.
 namespace async_mqtt5 {
 
+// Specify that the TLS handshake will be performed as a client.
 template <typename StreamBase>
 struct tls_handshake_type<boost::asio::ssl::stream<StreamBase>> {
 	static constexpr auto client = boost::asio::ssl::stream_base::client;
-	static constexpr auto server = boost::asio::ssl::stream_base::server;
 };
 
 // This client uses this function to indicate which hostname it is
@@ -37,49 +46,49 @@ void assign_tls_sni(
 
 } // end namespace async_mqtt5
 
-// The certificate file in the PEM format.
-constexpr char ca_cert[] =
-"-----BEGIN CERTIFICATE-----\n"
-"...........................\n"
-"-----END CERTIFICATE-----\n"
-;
+int main(int argc, char** argv) {
+	config cfg;
+
+	if (argc == 4) {
+		cfg.brokers = argv[1];
+		cfg.port = uint16_t(std::stoi(argv[2]));
+		cfg.client_id = argv[3];
+	}
 
-int main() {
 	boost::asio::io_context ioc;
 
-	// Context satisfying ``__TlsContext__`` requirements that the underlying SSL stream will use.
+	// TLS context that the underlying SSL stream will use.
 	// The purpose of the context is to allow us to set up TLS/SSL-related options. 
 	// See ``__SSL__`` for more information and options.
 	boost::asio::ssl::context context(boost::asio::ssl::context::tls_client);
 
-	async_mqtt5::error_code ec;
-
-	// Add the trusted certificate authority for performing verification.
-	context.add_certificate_authority(boost::asio::buffer(ca_cert), ec);
-	if (ec)
-		std::cout << "Failed to add certificate authority!" << std::endl;
-	ec.clear();
-
-	// Set peer verification mode used by the context.
-	// This will verify that the server's certificate is valid and signed by a trusted certificate authority.
-	context.set_verify_mode(boost::asio::ssl::verify_peer, ec);
-	if (ec)
-		std::cout << "Failed to set peer verification mode!" << std::endl;
-	ec.clear();
+	// Set up the TLS context.
+	// This step is highly dependent on the specific requirements of the Broker you are connecting to.
+	// Each broker may have its own standards and expectations for establishing a secure TLS/SSL connection. 
+	// This can include verifying certificates, setting up private keys, PSK authentication, and others.
 
 	// Construct the Client with ``__SSL_STREAM__`` as the underlying stream
-	// with ``__SSL_CONTEXT__`` as the ``__TlsContext__`` type.
+	// with ``__SSL_CONTEXT__`` as the ``__TlsContext__`` type
+	// and logging enabled.
 	async_mqtt5::mqtt_client<
 		boost::asio::ssl::stream<boost::asio::ip::tcp::socket>,
-		boost::asio::ssl::context
-	> client(ioc, std::move(context));
+		boost::asio::ssl::context,
+		async_mqtt5::logger
+	> client(ioc, std::move(context), async_mqtt5::logger(async_mqtt5::log_level::info));
 
-	// 8883 is the default TLS MQTT port.
-	client.brokers("<your-mqtt-broker>", 8883)
-		.async_run(boost::asio::detached);
+
+	// If you want to use the Client without logging, initialise it with the following line instead.
+	//async_mqtt5::mqtt_client<
+	//	boost::asio::ssl::stream<boost::asio::ip::tcp::socket>,
+	//	boost::asio::ssl::context
+	//> client(ioc, std::move(context));
+
+	client.brokers(cfg.brokers, cfg.port) // Broker that we want to connect to.
+		.credentials(cfg.client_id) // Set the Client Identifier. (optional)
+		.async_run(boost::asio::detached); // Start the Client.
 
 	client.async_publish<async_mqtt5::qos_e::at_most_once>(
-		"<topic>", "Hello world!",
+		"async-mqtt5/test", "Hello world!",
 		async_mqtt5::retain_e::no, async_mqtt5::publish_props{},
 		[&client](async_mqtt5::error_code ec) {
 			std::cout << ec.message() << std::endl;