Perfmon-Hook-Task-4 Implement PerfMonMgr Basics - start up, configuration

Complete Hook Task 4: Implement PerfMonMgr Basics - start up, configuration. This creates the initial PerfMonMgr class with stub functions. It should be able to parse configuration but not yet provide data processing.

See https://gitlab.isc.org/isc-projects/kea/-/wikis/Designs/performance-monitor#perfmon-hook-tasks

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