LoggerType concept

LoggerType represents an object type that will be used by the mqtt_client to log events.

A type satisfies the LoggerType concept if it defines any number (including zero) of the following functions:

Function signature	Arguments	Description
`void at_resolve(error_code ec, std::string_view host, std::string_view port, const asio::ip::tcp::resolver::results_type& eps);`	`ec` is the `error_code` returned by the resolve operation. `host` is the hostname used in the resolve. `port` is the port used in the resolve. `eps` is a list of endpoints returned by the resolve operation.	Invoked when the resolve operation is complete.
`void at_tcp_connect(error_code ec, asio::ip::tcp::endpoint ep);`	`ec` is the `error_code` returned by the TCP connect operation. `ep` is a TCP endpoint used to establish the TCP connection.	Invoked when the TCP connect operation is complete.
`void at_tls_handshake(error_code ec, asio::ip::tcp::endpoint ep);`	`ec` is the `error_code` returned by the the TLS handshake operation. `ep` is a TCP endpoint used to establish the TLS handshake.	Invoked when the TLS handshake operation is complete.
`void at_ws_handshake(error_code ec, asio::ip::tcp::endpoint ep);`	`ec` is the `error_code` returned by the the WebSocket handshake operation. `ep` is a TCP endpoint used to establish the WebSocket handshake.	Invoked when the WebSocket handshake operation is complete.
`void at_connack(reason_code rc, bool session_present, const connack_props& ca_props);`	`rc` is the `reason_code` received in the `CONNACK` packet indicating the result of the MQTT handshake. `session_present` A flag indicating whether the Broker already has a session associated with this connection. `ca_props` `async_mqtt5::connack_props` received in the `CONNACK` packet.	Invoked when the `CONNACK` packet is received, marking the completion of the MQTT handshake.
`void at_disconnect(reason_code rc, const disconnect_props& dc_props);`	`rc` is the `reason_code` received in the `DISCONNECT` packet specifying the reason behind the disconnection. `dc_props` `async_mqtt5::disconnect_props` received in the `DISCONNECT` packet.	Invoked when the `DISCONNECT` packet is received, indicating that the Broker wants to close this connection.

For example, a type T that defines at_connack and at_disconnect functions with their respective arguments is considered a valid LoggerType. This allows you to create your own LoggerType classes with functions of interest.

All defined functions are invoked directly within the mqtt_client using its default executor. If the mqtt_client is initialized with an explicit or implicit strand, none of the functions will be invoked concurrently.

	Warning
	Defined functions should not block and stop the `mqtt_client` from doing work.

A class that satifies this concept is logger.