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_tcp.cpp

@@ -7,27 +7,48 @@
 
 //[hello_world_over_tcp
 #include <iostream>
+#include <string>
 
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/detached.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 = 1883; // 1883 is the default TCP MQTT port.
+	std::string client_id = "async_mqtt5_tester";
+};
+
+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() {
-	//[init_tcp_client
-	// Initialize the execution context required to run I/O operations.
 	boost::asio::io_context ioc;
 
-	// Construct the Client with ``__TCP_SOCKET__`` as the underlying stream.
-	async_mqtt5::mqtt_client<boost::asio::ip::tcp::socket> client(ioc);
+	//[init_tcp_client_with_logger
+	// Construct the Client with ``__TCP_SOCKET__`` as the underlying stream and enabled logging.
+	// Since we are not establishing a secure connection, set the TlsContext template parameter to std::monostate.
+	async_mqtt5::mqtt_client<
+		boost::asio::ip::tcp::socket, std::monostate /* TlsContext */, async_mqtt5::logger
+	> client(ioc, {} /* tls_context */, async_mqtt5::logger(async_mqtt5::log_level::info));
 	//]
 
+	// If you want to use the Client without logging, initialise it with the following line instead.
+	//async_mqtt5::mqtt_client<boost::asio::ip::tcp::socket> client(ioc);
+
 	//[configure_tcp_client
-	// 1883 is the default TCP MQTT port.
-	client.brokers("broker.hivemq.com", 1883)
-		.credentials("async_mqtt5_tester")
-		.async_run(boost::asio::detached);
+	client.brokers(cfg.brokers, cfg.port) // Set the Broker to connect to.
+		.credentials(cfg.client_id) // Set the Client Identifier. (optional)
+		.async_run(boost::asio::detached); // Start the Client.
 	//]
 
 	//[publish_hello_world