fix: use sparkplugb decoder only for spBv1.0 topic

[bj00rn]

Dirty fix #792 by inspecting topic as suggested by https://github.com/thomasnordquist/MQTT-Explorer/issues/792#issuecomment-2111972106

Tested with sparkplug client https://github.com/eclipse/tahu/blob/master/javascript/examples/simple/example.js

The whole decoder choice logic should probably be cleaned up later on to support #493 and the likes. I guess decoding should not be done in backend in order to choose decoder interactively.. See #655