Mongoose MQTT client

This MQTT client implementation implements the following:

Connects to the MQTT server specified by MQTT_SERVER_URL
When connected, subscribes to the topic MQTT_SUBSCRIBE_TOPIC
When it receives a message, echoes it back to MQTT_PUBLISH_TOPIC
Timer-based reconnection logic revives the connection when it is down
Ping server periodically. When disconnected, a last will is published
Implements "ota.update" for OTA updates, see https://mongoose.ws/mqtt/

By default, it uses HiveMQ public broker, and can be tested with the HiveMQ Websocket Client. Connect, subscribe to mg/d3/#, send a message to mg/d3/rx.

Integrating into an embedded project

Copy mongoose_mqtt.c to your embedded project and add it to the build
Add mg_mqtt_init(&mgr) after mg_mgr_init()
Add mg_mqtt_poll(&mgr) after mg_mgr_poll()

EMQX

See full walk-trough at https://mongoose.ws/articles/stm32-emqx/

Amazon AWS IoT

Login to AWS console, choose AWS IoT
On the left menu, choose Connect / Domain configurations. Copy the domain name and update MQTT_SERVER_URL. Note to set "mqtts" and port 8883:
```
#define MQTT_SERVER_URL "mqtts://COPIED_DOMAIN:8883"
#define MQTT_CLIENT_ID "d1"
```

Create EC CSR for device certificate:

openssl req -new -newkey ec -pkeyopt ec_paramgen_curve:prime256v1   -pkeyopt ec_param_enc:named_curve -nodes   -keyout device.key -out device.csr -subj /CN=d1

On the left menu, click on Manage / All devices / Things, Create things, Create single thing, Set thing name to d1, No shadow, Next, Upload CSR (use device.csr created on a previous step), Next, Create Policy, name "AllowAll", set both action and resouce to *, Create, Attach AllowAll to the certificate, Create Thing.
Click on Things / d1 / Certificates, make the certificate active
Click on the certificate, download it, rename to "device.crt"
Set TLS_CA. Visit https://mongoose.ws/tls/, enter DOMAIN_NAME:8883 into the CA field and click on the "Get CA Certificate" button. Enable the "Show as C/C++ constant", copy-paste to your code.
Set TLS_KEY and TLS_CRT with the output of this commands:

sed 's/\r$//; s/.*/  "&\\n"/; $!s/$/ \\/' device.key
sed 's/\r$//; s/.*/  "&\\n"/; $!s/$/ \\/' device.crt

Click on Test / MQTT test client, subscribe to topic mg/#, send a message to mg/d1/rx

Microsoft Azure IoT Hub

See full walk-trough at https://mongoose.ws/articles/stm32-azure-iot-hub/

Create IoT Hub
Generate device self-signed certificates:

openssl req -x509 -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -pkeyopt ec_param_enc:named_curve -nodes -keyout device.key -out device.crt -days 3650 -subj "/CN=MYDEVICE"

Show certificate thumbprint:

openssl x509 -in device.crt -noout -fingerprint -sha256 | tr -d ':' | cut -d= -f2

#define AZURE_HUB_NAME "HUB_NAME"    // Change this
#define AZURE_DEVICE_ID "DEVICE_ID"  // Change this

// Do not change this
#define MQTT_SERVER_URL "mqtts://" AZURE_HUB_NAME ".device.azure-devices.net"
#define MQTT_CLIENT_ID AZURE_DEVICE_ID
#define MQTT_USER AZURE_HUB_NAME ".azure-devices.net/" AZURE_DEVICE_ID "/?api-version=2021-04-12"
#define MQTT_PUBLISH_TOPIC "devices/" AZURE_DEVICE_ID "/messages/events/"
#define MQTT_SUBSCRIBE_TOPIC "devices/" AZURE_DEVICE_ID "/messages/devicebound/#"

Set TLS_CA. Visit https://mongoose.ws/tls/, enter "HUB_NAME.device.azure-devices.net:8883" into the CA field and click on the "Get CA Certificate" button. Enable the "Show as C/C++ constant", copy-paste to your code.
Set TLS_KEY and TLS_CRT with the output of this commands:

sed 's/\r$//; s/.*/  "&\\n"/; $!s/$/ \\/' device.key
sed 's/\r$//; s/.*/  "&\\n"/; $!s/$/ \\/' device.crt

Add to your mongoose_config.h:

#define MG_ENABLE_CHACHA20 0

Microsoft Azure Event Grid

Create Azure Event Grid (EG) instance
Generate device self-signed certificates:

openssl req -x509 -newkey ec -pkeyopt ec_paramgen_curve:prime256v1 -pkeyopt ec_param_enc:named_curve -nodes -keyout device.key -out device.crt -days 3650 -subj "/CN=MYDEVICE"

Show certificate thumbprint:

openssl x509 -in device.crt -noout -fingerprint -sha256 | tr -d ':' | cut -d= -f2

Register a client on EG: choose a name, and use the thumbprint from above
Set URL, username and client ID:

#define MQTT_SERVER_URL "mqtts://INSTANCE.REGION.ts.eventgrid.azure.net:8883"
#define MQTT_CLIENT_ID "CLIENT_NAME"
#define MQTT_USER "CLIENT_NAME"

Set TLS_CA. Visit https://mongoose.ws/tls/, enter "INSTANCE.REGION.ts.eventgrid.azure.net:8883" into the CA field and click on the "Get CA Certificate" button. Enable the "Show as C/C++ constant", copy-paste to your code.
Set TLS_KEY and TLS_CRT with the output of this commands:

sed 's/\r$//; s/.*/  "&\\n"/; $!s/$/ \\/' device.key
sed 's/\r$//; s/.*/  "&\\n"/; $!s/$/ \\/' device.crt

In Azure EG, go to Instance / MQTT Broker / Topic spaces, add "space1" with mg/# pattern
In Azure EG, go to Instance / MQTT Broker / Permissions bindings, add binding1 and binding2:

$all , space1 , Publisher
$all , space1 , Subscriber