Mqtt observable property is not exposed

[hasanheroglu]

https://github.com/eclipse/thingweb.node-wot/blob/09dfd6ab03805b7b39ab39433880496466d25e67/packages/binding-mqtt/src/mqtt-broker-server.ts#L123-L149

In case property has observable: true and writeOnly: true fields form for observable not exposed. Additionally, if readOnly: true at the same time, no forms would be exposed.

It could be better to have the code like this:

if (property.observable) {
    // observable form expose logic
}
if (property.writeOnly) {
    // writeOnly form expose logic
}

And I think, we can skip totally readOnly since readproperty is not used with mqtt.