Allow modifying datanode jvm.options

There is currently no easy way how to adapt jvm.options content used for opensearch in datanode. This is partly by purpose - making sure that opensearch is configured correctly for datanode needs. But it also prevents configuration of opensearch tmp path and GC log file location.

During bootstrap config sync datanode will copy https://github.com/Graylog2/graylog2-server/blob/master/data-node/src/main/resources/opensearch/config/jvm.options to the opensearch config dir. From there, opensearch will read and use it. In distributed packages, this file is part of the datanode jar, impossible to modify.

## Why?
In the jvm.options there are some hardcoded configurations, like GC configuration and its log file location. Also opensearch temp dir is not easily configurable, except propagating `OPENSEARCH_TMPDIR` env property through datanode.

Requested in https://community.graylog.org/t/how-do-i-change-the-gc-log-and-jvm-temp-files-location-from-my-datanode/35620