[RFC-0010] Introduce object-level workload identity for ImageRepository

[matheuscscp]

Depends on: fluxcd/pkg#919

Part of: fluxcd/flux2#5022

This PR also introduces a token cache for caching the cloud provider access tokens. This token cache will report the following metrics:

• Counter gotk_token_cache_events_total
  • Help: Total number of cache retrieval events for a Gitops Toolkit resource reconciliation.
  • Labels: event_type, kind, name, namespace, operation. The values of event_type can be cache_miss or cache_hit. The value of operation is always reconcile.
• Counter gotk_token_cache_requests_total
  • Help: Total number of cache requests partioned by success or failure.
  • Labels: status. The values of status can be success or failure.
• Counter gotk_token_cache_evictions_total
  • Help: Total number of cache evictions.
  • No labels.
• Gauge gotk_token_cached_items
  • Help: Total number of items in the cache.
  • No labels.

I tested this PR in the following setups:

AWS:

• Controller-level
• Object-level

Azure:

• Controller-level
• Object-level

GCP:

• Controller-level with GCP service account impersonation
• Object-level with GCP service account impersonation
• Object-level with direct access

[stefanprodan]

LGTM

Thanks @matheuscscp 🏅