Feature: make LWT Retain flag optional, Azure event grid compatibility

[fenax]

Hello,

We would like to use chirpstack-mqtt-forwarder with Azure Event Grid as our MQTT broker.

Azure Event Grid does not support the retain flag (yet) but chirpstack-mqtt-forwarder uses it on the LWT message.

Would it be possible to make the use of the retain flag configurable ?

If I were to submit a pull request for it, would it be accepted ?

Thanks.