Consider to load jar extensions automatically from the working directory

[mikir]

Is your feature request related to a problem? Please describe.
Currently, there is only one way how to run independent external jar extension. For example, new Zserio C++17 extension can be run by the following command:

java -cp zserio.jar:zserio_cpp17.jar zserio.tools.ZserioTool schema_name.zs -cpp17 output_directory_name

This is cumbersome and not intuitive. The intuitive command could be

java -cp zserio_cpp17.jar -jar zserio.jar schema_name.zs -cpp17 output_directory_name

However, this command is not supported by Java engine. Option -cp is ignored if option -java is used.

Describe the solution you'd like
It would be probably better if Zserio core will automatically try to load all jar extensions available in the working directory (directory where the zserio.jar is located). Then, it would be for example enough just to copy binary zserio_cpp17.jar into the directory where zserio.jar is located and run the same command as usual:

java -jar zserio.jar  schema_name.zs -cpp17 output_directory_name

Describe alternatives you've considered
We might consider to introduce new command line option which will specify the directory where to search for Zserio jar extensions and not to load them automatically. This would give users the option to disable automatic extension loading. However, this will require a little bit more complicated command line. Example:

java -jar zserio.jar -extensionsDir extensions_directory schema_name.zs -cpp17 output_directory_name