Ability to set custom clientId

[Zoltán Borsos]

Is your feature request related to a problem? Please describe.
My MQTT server requires a specific clientID for each connected client and user (VerneMQ), and also i think the MQTT5 specification require a fixed clientID.

Describe the solution you'd like
Add a new input parameter where i able to specify a custom clientID, a look at the MQTTNet package documentation and the library is capable to handle it, trough the MqttClientOptionsBuilder::withClientId() method. We only need to call this method when the input is specified, otherwise a random clientID gets generated on initialization.

Describe alternatives you've considered
I could turn on a settings in VerneMQ config that disables this specific clientID check, but this option is already deprecated and gets removed in the next major release.