Support outgoing connections

[rechrtb]

Overview

RRF-side support for creation of outgoing connections (see Duet3D/WiFiSocketServerRTOS#42).

As a demonstration, connecting to an MQTT broker and publishing messages via M118 is implemented. The RRF MQTT client is implemented using MQTT-C.

The MQTT client can be configured using the M586 command. The designated MQTT protocol number is 5. It adds a new parameter H to specify the broker IP address.

A new fractional command M586.X is also introduced (X = 5 for MQTT, can be used for other client protocols in the future). This allows further configuration of the MQTT client like setting the username, password, etc.

• C - Client ID
• U, K - Username and password ()
• S, Q - Subcription topic and corresponding QOS
• P, D, R, Q - Publish settings: topic, duplicate flag, retain flag, QOS
• W, T - Will message and topic

To publish an MQTT message via M118, use parameter P6.

A small demonstration is described in Tools/mqttecho/README.md

Code/Data Size

Static:

Without PR:

   text    data     bss     dec     hex filename
 760824     328  101808  862960   d2af0 Duet3Mini5plus/Duet3Firmware_Mini5plus.elf

With PR:

   text    data     bss     dec     hex filename
 771400     336  101872  873608   d5488 Duet3Mini5plus/Duet3Firmware_Mini5plus.elf

Dynamic:

Creating an MqttClient object allocates 2K and 1K for send and receive buffers. Memory for username, password, subscriptions, publish topic, client ID, will message and topic are allocated dynamically and whose sizes depend on user-given values through M586.X.

[PaulG4H]

@rechrtb maybe you are so kind and improve the mqtt functionality as shown here:
#880

Many thank's for your great work!