fix(cmd): do not override `kong.conf` if `--db-timeout` is not passed

[ms2008]

Summary

Prior to this fix, if the gateway was started using the kong cli, the final pg_timeout would be overridden to 60s instead of the value specified in kong.conf, even if --db-timeout was not explicitly passed in. This was fixed by making cli args --db-timeout optional, so that only explicitly passed in arguments would override the pg_timeout otherwise the value specified in kong.conf would continue to be used.

Checklist

• [ ] The Pull Request has tests
• [x] A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• [ ] There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix https://github.com/Kong/kong/discussions/10307#discussioncomment-5405050