feat(e2e): expose _applyAppConfig

[AndrienkoAleksandr]

Expose _applyAppConfig to make it reusable with rolloutRestart and scaleDownAndRestart. For example e2e test writer could make working such patched RHDH configuration:

      await rhdh.configure({
        auth: "keycloak",
        appConfig: "tests/config/app-config-rhdh-with-default-permissions.yaml", // changed configuration
        valueFile: "tests/config/values.yaml",
      });
      await rhdh.applyAppConfig();
      await rhdh.scaleDownAndRestart();
      await rhdh.waitUntilReady();