fix(emqx_gateway): convert and clear authentication certificates

[SergeTupchiy]

Summary

🤖 Generated by Copilot at b98a716

This pull request fixes a memory leak issue in the emqx_gateway module by storing the TLS certificates and keys for the gateway authentication mechanisms in the data directory instead of in memory. It also adds some helper functions in the emqx_authentication_config module to handle the certificate and key files. The change is documented in the changes/ce/fix-10653.en.md file.

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

[yanzhiemq]

Bug Fixes

• Stored gateway authentication TLS certificates and keys in the data directory to fix the problem of memory leakage.

[yanzhiemq]

修复

• 将网关认证的 TLS 证书和密钥保存到数据目录以修复内存泄漏问题。