Introduce `pds.debug.enabled` + debug new type "DEBUG" for sechub messages

[de-jcup]

Situation

It is difficult to write and debug new PDS solution, because we always have to inspect the output stream for details etc.

Wanted

A simple way to send debug information back via existing back channel (sechub user messages)

Solution

• we introduce new SecHub message type "debug"
• we introduce "debugMessge" in our test scripts (+ documentation) which write "DEBUG_...txt" but only when env variable "PDS_DEBUG_ENABLED" is "true"
• we accept "DEBUG_" as prefix for type inspection
• we introduce a new default PDS parameter pds.debug.enabled which is per default false