[module-sdk] expose readonly Settings on ApplicationHookInput

[Fkuloff]

Adds a readonly Settings field to ApplicationHookInput, symmetrical to how module hooks receive ConfigValues. Typed as pkg.ReadableValuesCollector (no Set/Remove). Reuses the existing CONFIG_VALUES_PATH channel — no new env var, no addon-operator changes.

Example hook:

func Handle(_ context.Context, input *pkg.ApplicationHookInput) error {
    enabled, ok := input.Settings.GetOk("apiServersDiscovery.enabled")
    if !ok || !enabled.Bool() {
        input.Logger.Info("apiServersDiscovery.enabled is not set — skipping")
        return nil
    }
    // ...discover apiserver pods and write into values
}

Unit test in main_test.go covers both branches (gate closed → Values.Set not called; gate open → called with expected pods).

Cluster-verified on a live Deckhouse cluster via artem-test, gating on triggersEnabled:

// gate closed (default false)
{"msg":"triggersEnabled is not set — skipping","hook_module":"default.test","time":"2026-04-22T15:36:53Z"}

// gate open (after k patch ... triggersEnabled:true + matching pod)
{"msg":"discover triggers","hook_module":"default.test","time":"2026-04-22T15:40:43Z"}
{"msg":"discovered triggers","hook_pods":["trigger-demo"],"hook_module":"default.test","time":"2026-04-22T15:40:43Z"}