Add port for NETX use

[JacobBarthelmeh]

Adds example port for NETX use and adds the macro EXTERNAL_MQTT_TLS_CALLBACK for compiling out the example TLS callback function.

[Copilot]

Pull Request Overview

This pull request adds support for NETX by introducing NETX-specific macros, new fields in the SocketContext, and backend functions for handling socket operations via NETX. It also adds conditional compilation to allow external MQTT TLS callback implementations.

• Added NETX configuration and macro definitions in mqttport.h.
• Extended SocketContext with NETX fields and implemented NETX backend functions in mqttnet.c.
• Adjusted compilation conditionals for MQTT TLS in mqttexample.h and mqttexample.c.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
examples/mqttport.h	Added NETX-specific include and macro definitions.
examples/mqttnet.h	Extended SocketContext with NETX-related members.
examples/mqttnet.c	Introduced NETX backend functions (disconnect, connect, write, read) and adjusted socket initialization.
examples/mqttexample.h	Added conditional exclusion based on EXTERNAL_MQTT_TLS_CALLBACK.
examples/mqttexample.c	Modified TLS callback conditional compilation.