Added custom truststore setup in entrypoint

[JustInVTime]

When running Graylog in a kubernetes environment using the OpenSearch operator, it's mandatory to add the OpenSearch CA to the truststore.

By mounting certificates in the /certificates folder, this script will create a custom trust store based on the default trust store and add the custom certificates during entrypoint execution.

Notes for Reviewers

• The commit history must be preserved - please use the rebase-merge or standard merge option instead of squash-merge
• Sync up with the author before merging

[CLAassistant]

All committers have signed the CLA.

[bernd]

Thank you! 🙏