Fixed HexToBytes function. Added Publish3 command to send binary data via MQTT.

[mbezuidenhout]

Description:

Add ability to the Publish command to send a hexstring as binary MQTT data.

Publish command now has the following options

Publish3 | = MQTT publish any topic and optional binary payload. For example the payload 7461736D6F7461 will output tasmota or the value ff will output the integer value 255

Checklist:

• [x] The pull request is done against the latest development branch
• [x] Only relevant files were touched
• [x] Only one feature/fix was added per PR and the code change compiles without warnings
• [x] The code change is tested and works with Tasmota core ESP8266 V.2.7.6
• [x] The code change is tested and works with Tasmota core ESP32 V.3.0.0
• [x] I accept the CLA.

[Jason2866]

A niche function which needs resources in every build, not a fan of.

[s-hadinger]

We could consider excluding this feature in SAFEBOOT as it shouldn't be useful there

[Jason2866]

We could consider excluding this feature in SAFEBOOT as it shouldn't be useful there

Fine when doing this way.