How to add per-message properties

[rmja]

I would like to add the Property::ContentType to the mqtt messages. connect_to_broker() does not include the property as property_allowed() returns false for that property and send_message() does not include any properties from the config.
Ideally there should be some way of specifying per-message properties - is this currently possible? I may be missing something.

[rmja]

One option would be to return some kind of builder when invoking send_message() like reqwless does https://github.com/drogue-iot/reqwless/blob/main/src/client.rs

[obabec]

Hi @rmja feel free to create PR :)