Cache main OCI packages when running with `kcl run oci://...`

[kptranum]

Feature Request

Is your feature request related to a problem? Please describe:
When executing remote OCI-hosted KCL packages directly using kcl run oci://<registry>/<repo>:<tag>, KCL downloads the package on every invocation. Unlike dependencies declared in a kcl.mod file (which are cached by KPM), the main package itself is treated as ephemeral. This causes:

• Repeated network downloads (performance penalty)
• Higher load on private registries
• Inability to run previously used packages offline
• Inefficiencies in controller/operator contexts (e.g. Crossplane reconciliation loops)

Concrete pain point (Crossplane):
In a Crossplane Composition using crossplane-contrib/function-kcl, the function is invoked during every reconciliation cycle. A configuration like:

input:
  apiVersion: template.fn.crossplane.io/v1beta1
  kind: KCLInput
  metadata:
    name: test
  spec:
    source: oci://<registry>/<repo>:<tag>

leads to the OCI package being re-downloaded on each reconciliation, effectively hammering the registry and adding latency.

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Describe the feature you'd like:
Add persistent caching for the primary OCI package referenced by kcl run oci://..., similar to how:

• Docker caches images after the first pull
• KPM caches declared dependencies in kcl.mod

Desired behavior:

1. On first execution, download and store the referenced <registry>/<repo>:<tag> (or digest) in the existing KPM cache (e.g. ~/.kcl/kpm/...).
2. On subsequent identical references (same tag or digest), reuse the cached artifact with zero network fetch.
3. Provide optional flags:
   • --no-cache or --refresh to force a re-download
   • --cache-info (or similar) to show whether a cached artifact was used

---

Describe alternatives you've considered:
???

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Teachability, Documentation, Adoption, Migration Strategy:

Scenarios improved:

• Crossplane compositions: Reconciliation loops no longer trigger repeated downloads; faster convergence and reduced registry load.
• CI pipelines: Multiple jobs referencing the same OCI package avoid redundant network I/O.
• Developer workflows: Faster iterative runs when testing remote packages.
• Offline / air-gapped: A package can be “primed” once, then reused offline.
• Reproducibility: Digest-based caching encourages immutable references.

Documentation updates:

• Add a “Main Package Caching” section under “Running Remote Packages”.
• Update kcl run --help with new flags (--no-cache, --refresh).
• Provide troubleshooting guidance: how to inspect/clear the cache.
• Best practice note: prefer digest references for guaranteed immutability.

Backward compatibility:

• Default behavior silently improves performance (no flag required).
• Users needing the current always-fetch semantics can opt out with --no-cache or an env var.

No migration blockers—this is an additive optimization.