Cassandra CQL exporter - export multiple keyspaces

Just contributed to the great Cassandra CQL exporter tool on github:
List of my changes:
- added possibility to export multiple keyspaces separated with ","
- added possibility to export keyspaces specified in a file
- added gradle build
- updated README.md with gradle build instructions and updated parameter description "--secure", "-k" "-kf
I hope that the author will merge that pull request :)
Matej Koval is a Full stack senior Java developer, Scrum master of a team.
Experienced in Debian Linux administration and technologies like: Spring Boot, Quarkus, Kafka, Docker, Kubernetes, Angular.
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