Scripting: Add configuration parameter to control execution of indexed s...

...cripts.

This commit adds the `script.disable_indexed` parameter to the config system.
If this is set to `true` indexed scripts cannot be created or used at search time.
If this is set to `sandbox` only indexed script that use sandboxed languages can be created or
used at search time.
If this is set to `false` indexed scripts from any supported language can be created or used at
search time.
The default value is `sandbox`.
This setting is independent of the `script.disable_dynamic` setting.

See #6922
How can you use indexed Java scripts? Surely the only way to use native scripts is by putting them into your classpath?