Refactor sampleexternalplugin to be a Valid Reference Implementation

[nerdeveloper]

Summary

This PR refactors the sampleexternalplugin from outdated mock scaffolding into a realistic, working reference implementation that demonstrates best practices for external plugin development.

Problem

The current sampleexternalplugin (issue #4824) had several issues:

• ❌ Mock scaffolding that created useless text files (initFile.txt, apiFile.txt)
• ❌ Didn't use PROJECT config or align with internal plugin architecture
• ❌ Used release module instead of local source for testing
• ❌ Not a realistic example for plugin developers

Solution

Transformed the plugin into a Prometheus Monitoring Generator that:

• ✅ Uses edit subcommand to add optional Prometheus monitoring features
• ✅ Reads PROJECT config for domain and project metadata
• ✅ Generates real ServiceMonitor YAML manifests
• ✅ Uses local Kubebuilder source via replace directive
• ✅ Follows internal plugin architecture patterns

Changes

Plugin Functionality

• Removed: Mock init, create api, create webhook subcommands
• Added: Realistic edit subcommand that scaffolds Prometheus monitoring
• Implements: PROJECT config reading (domain, projectName, repo)
• Generates:
  • config/prometheus/monitor.yaml - ServiceMonitor resource
  • config/prometheus/kustomization.yaml - Kustomize config
  • config/default/kustomization_prometheus_patch.yaml - Integration instructions

Code Quality

• Uses config.Config APIs like internal plugins
• Proper Universe pattern for file scaffolding
• Local source replace directive: replace sigs.k8s.io/kubebuilder/v4 => ../../../../../../../

Documentation

• Updated external-plugins.md with realistic edit command examples
• Changed examples from mock init/api/webhook to real Prometheus monitoring use case

Usage Example

# Add Prometheus monitoring to your operator project
kubebuilder edit --plugins sampleexternalplugin/v1

This will scaffold:

• Prometheus ServiceMonitor configuration
• Kustomize resources for monitoring
• Integration instructions

Testing

• ✅ Plugin builds successfully with local source
• ✅ All mock files removed, real templates in place
• ✅ Documentation reflects new usage

Impact

This PR makes the sample plugin a proper reference that:

• Shows how to read PROJECT config in external plugins
• Demonstrates real-world use case (optional monitoring features)
• Allows contributors to test against local Kubebuilder source
• Aligns with internal plugin architecture patterns

Fixes #4824

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[vitorfloriano]

Hi @nerdeveloper thanks for the PR! This is a large one and will take some time to review. While you wait for the review, could you please adjust the title so it follows the guidelines for PR titles? Also, could you squash the four commits into one just so we keep the rule of a single commit per PR? Thanks!