feat: introduce top-level plugins list command for active scalibr plugins

[techleadevelopers]

Description

This PR introduces a new top-level CLI command plugins list to address the missing capability of inspecting active/installed plugins directly via the CLI (resolving the capability requested in #2783).

Instead of adding complex output formats, the command is built as a lightweight, clean entry point that interfaces with the underlying plugin catalog helper layer.

Changes

• cmd/osv-scanner/main.go & testmain_test.go: Registered the new plugins command group and wired dependencies.
• cmd/osv-scanner/plugins/: Created the new sub-command structure (command.go), integration tests (command_test.go), and testing bootstraps.
• internal/scalibrplugin/: Implemented resolve.go and its respective tests to cleanly fetch and expose the active plugin list from the catalog without bleeding internal structures into the CLI logic.

Testing Strategy

• Added unit tests for the plugin resolution helper logic under internal/scalibrplugin.
• Added integration/CLI validation tests in cmd/osv-scanner/plugins/command_test.go using pure content-matching assertions instead of strict file-based snapshots to reduce future CI/CD friction and ensure test stability.

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[G-Rath]

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