support setting extended client options

[hotraygroup]

support setting extended client options via callback

[gen2thomas]

Hi @hotraygroup thanks for adjustments. Some comments:

• please add a small unit test for the new functionality, at least for "Connect()"
• is there the possibility to show the usage of the new functionality in an example?

For the example: There are 3 at the moment. Either you can add to one of them or write a new one.

[gen2thomas]

accidentally closed - now reopen

[gen2thomas]

Hi @hotraygroup , I'm going to set this PR back to draft since there is no progress yet. Feel free to fix the remarks and set it back to active.

[hotraygroup]

usage: mqttAdaptor := mqtt.NewAdaptorWithAuth(...) mqttAdaptor.SetOptionsFn(func(opts *paho.ClientOptions) {opts.WillPayload = []byte("goodbye")})