Add Temp Hidden Agent Dir

[aphralG]

Proposed changes

During a config apply agent will create a temp hidden directory .agent_<agent_id> at the location of the nginx.conf which will contain the temp directories for atomic writes and rollback

When an NGINXConfigContext update occurs agent will check if this directory exists if it does it will be removed this fixes the scenario where agent could fail to clean up the directories after a config apply because it was restarted.

Checklist

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• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13