Disable scripted UDFs by default

(Follow-up to CASSANDRA-9402)

TL;DR this ticket is about to add an other config option to enable scripted UDFs.

Securing Java-UDFs is much easier than scripted UDFs.

The secure execution of scripted UDFs heavily relies on "how secure" a particular script provider implementation is. Nashorn is probably pretty good at this - but (as discussed offline with iamaleksey) we are not certain. This becomes worse with other JSR-223 providers (which need to be installed by the user anyway).

E.g.:

# Enables use of scripted UDFs.
# Java UDFs are always enabled, if enable_user_defined_functions is true.
# Enable this option to be able to use UDFs with "language javascript" or any custom JSR-223 provider.
enable_scripted_user_defined_functions: false

TBH: I would feel more comfortable to have this one. But we should review this along with enable_user_defined_functions for 4.0.