[voicerss] Fix and enhance the external tool to prefill the cache (#12155)

[voicerss] Fix and enhance the external tool to prefill the cache (#12155)

* [voicerss] Fix and enhance the external tool to prefill the cache
* Make the tool independent of core openHAB
* Fix the parsing of the command arguments.
* Allow to provide optional audio codec and format.
* Null annotations added on most of the classes
* Simplified doc about classpath
* Suppress obsolete mention of open issue in doc

Signed-off-by: Laurent Garnier <lg.hc@free.fr>