feat: allow dynamic loading of ArgoCD URLs from secrets

[ksawerykarwacki]

Summary

Add support for dynamically loading ArgoCD shard URLs from Kubernetes secrets without requiring Helm chart redeployment.

• Secrets labeled with kargo.akuity.io/argocd-shard=true in the Kargo namespace are watched and used to configure ArgoCD shard URLs
• Dynamic shards override static (Helm) configuration with the same name
• When a secret is deleted, any static configuration is restored
• Backwards compatible - existing Helm configuration continues to work unchanged

Secret Format

apiVersion: v1
kind: Secret
metadata:
  name: argocd-shard-production
  namespace: kargo
  labels:
    kargo.akuity.io/argocd-shard: "true"
type: Opaque
stringData:
  name: "production"                      # Shard identifier (empty for default)
  url: "https://argocd.example.com"       # ArgoCD base URL

Closes #4971

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[ksawerykarwacki]

Any chances that soemone will look at this? @krancour, @hiddeco?

This is small change but lack of it is rather frustrating in the sharded deployement.