Add possibility to set a custom MQTT client Id in VrpcAgent

[cstim]

Is your feature request related to a problem? Please describe.
When the VrpcAgent object starts the MQTT connection, it also sets a MQTT client ID. This client ID is generated in the VrpcAgent.js code but cannot be set from the application code. Sometimes the application needs to set the MQTT client ID into some controlled string before the MQTT connection is established, so it should be possible to set this.

Describe the solution you'd like
Add an optional mqttClientId argument to VrpcAgent.serve(), or as option into the constructor.

Related: In VrpcClient there also isn't an argument or option, but the mqtt client id is set in the constructor as member variable this._mqttClientId already, so the application code can just modify that member variable before calling connect. Not particularly elegant, but works already.

[bheisen]

Yes, I believe that you are right. I will add the corresponding option, both for the agent and the client.

[bheisen]

I think I like the option in the constructor better and keep the serve() function free from any configuration. Will hence provide an own PR soonish. Thank you for your PR anyways!

[create-issue-branch]

Branch feature/56-add-possibility-to-set-a-custom-mqtt-client-id-in-vrpcagent created!