feat: implement AppService control plane proxy with TTL cache

[AdilFayyaz]

Depends on: #7175

Why are the changes needed?

The control plane needs a stable API surface for app management without making direct K8s calls. AppService proxies to InternalAppService (data plane) and adds a TTL cache to reduce cross-plane RPC calls on hot Get paths.

What changes were proposed in this pull request?

• Added app/service/AppService — cache-aside proxy over InternalAppService; Get reads from cache, mutations invalidate, Watch is a streaming passthrough
• Added app/setup.go — mounts AppService on the control plane mux; in unified mode routes to /internal on the same mux with no network hop
• Restructured config: app/internal/config/InternalAppConfig (data plane) and app/config/AppConfig (control plane, holds CacheTTL + InternalAppServiceURL)
• Removed AppService stub and wiring from runs/ — no longer needed there

How was this patch tested?

• go test ./app/service/... — 8 unit tests covering cache hit, miss, TTL expiry, invalidation on Create/Update/Delete, List always forwarding, and Watch stream proxying
• go test ./app/internal/service/... — covers InternalAppService RPCs including DesiredState branching, pagination, and Watch initial snapshot
• go test ./app/... — all packages pass

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

• main
  • Flyte 2 WIP #6583
    • feat: add AppK8sClient for KService lifecycle management #7166
      • feat: implement InternalAppService for Knative app lifecycle #7175
        • feat: implement AppService control plane proxy with TTL cache 👈
          • feat: resilient Watch RPC with reconnect and resourceVersion tracking #7188
            • feat: path based ingress routing for Flyte apps via Traefik + Knative #7197