[S7] Implement connection closing for S7 protocol

It seems I skipped porting the code to gracefully close a connection in the transition from 0.6 to 0.7. This is a great low-hanging fruit, so I'll leave this here for someone to picup.

We already generally have the parts in place, however they are not quite correct:

https://github.com/apache/plc4x/blob/develop/protocols/s7/src/main/resources/protocols/s7/s7.mspec

Defines a type: COTPPacketDisconnectRequest, however the third parameter is not class, but disconnectReason.

The old TPDU is defined here:
https://github.com/apache/plc4x/blob/rel/0.6/plc4j/protocols/iso-tp/src/main/java/org/apache/plc4x/java/isotp/protocol/model/tpdus/DisconnectRequestTpdu.java

This should be an enum type. The constant names and values can be taken from here:
https://github.com/apache/plc4x/blob/rel/0.6/plc4j/protocols/iso-tp/src/main/java/org/apache/plc4x/java/isotp/protocol/model/types/DisconnectReason.java

As soon as these changes are in place and the code has been generated, the logic for closing can be implemented by being inspired by the old drivers code:
https://github.com/apache/plc4x/blob/rel/0.6/plc4j/drivers/s7/src/main/java/org/apache/plc4x/java/s7/connection/S7PlcConnection.java