Expose the serial port's logger for RTU connections

[andig]

While the tcpclient's tcpTransporter exposes a logger, the rtuclient's rtuSerialTransporter embeds a private serialPort which has a logger but isn't externally accessible. This PR exposes the serial ports logger through the handler similar to the tcpTransporter.

[andig]

Noticed this when being unable to debug a serial connection issue in a bigger application. The change is backwards-compatible as the Logger field didn‘t exist before and the serial‘s logger was sealed away from access.

[guelfey]

LGTM, up to @gq0 and @frzifus

[andig]

Thanks @frzifus for the assignment but I'm afraid there's not much more I can do right now ;)

[gq0]

LGTM