Introduce Loader interface to support alternative event and metric initialization methods

[harp-intel]

The legacy (original) approach to loading events and metrics has been refactored under the new Loader interface and is found in loader_legacy.go.

A new approach that creates perf event groups dynamically at runtime from perfmon (https://github.com/intel/perfmon) event and metric definitions is in loader_perfmon*.go. This approach eliminates the need to manually create perf event groups found in the legacy approach.

The intent is for additional methods of loading events/metrics, e.g., for other architectures, to use the new Loader interface so as to avoid co-mingling the approaches in nested if/else code.

[Copilot]

Pull Request Overview

This PR introduces a Loader interface to support alternative event and metric initialization methods, transitioning from a monolithic legacy approach to a modular architecture that can accommodate different event/metric loading strategies.

• Refactors legacy event/metric loading approach under new Loader interface
• Introduces perfmon-based loader that dynamically creates perf event groups from Intel perfmon definitions
• Adds comprehensive perfmon configuration files for Intel CPU architectures (Sierra Forest, Sapphire Rapids, Ice Lake)

Reviewed Changes

Copilot reviewed 40 out of 68 changed files in this pull request and generated 42 comments.

File	Description
cmd/metrics/resources/perfmon/srf/srf.json	Adds perfmon configuration for Intel Xeon 6 processors with E-cores (Sierra Forest)
cmd/metrics/resources/perfmon/spr/spr.json	Adds perfmon configuration for Intel 4th Generation Xeon processors (Sapphire Rapids)
cmd/metrics/resources/perfmon/icx/icx.json	Adds perfmon configuration for Intel 3rd Generation Xeon processors (Ice Lake)