feat: configurable TCP keepalive

[terry-xiaoyu]

Fixes

Summary

🤖 Generated by Copilot at b548340

This pull request adds a new feature to configure TCP keepalive settings for MQTT connections, and improves the consistency, reliability, and testability of the code and configuration files. It modifies the etc/emqx.conf, priv/emqx.schema, src/emqx_connection.erl, src/emqx_zone.erl, and test/emqx_mqtt_SUITE.erl files to implement and test the feature. It also enhances the scripts/relup-base-packages.sh, scripts/macos-sign-binaries.sh, and scripts/split-config.escript files to support different package types, sign new binaries, and skip invalid configuration sections.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update