Add a extensible interface for adding udfclient wrapper script for debugging

[tkilias]

Background:

• Sometimes, we need to debug the udfclient running directly in the database.
• For that, we often need to run another binary besides the udfclient and need to store its output outside of the UDF, because UDF only have temporary storage
• A common way to do this via a wrapper script which runs instead of the udfclient and then start the additional binary and the udfclient
• We already did this few times and to redo it everytime is annoying, so it would be good to have an extensible interface inject such wrapper scripts
• https://github.com/exasol/script-languages-release/tree/enhancement/debug_wrapper contains an incomplete first implementation

see also #238