…elastic#223525)

## 📓 Summary

This work exposes an asynchronous
`coreSetup.pricing.isFeatureAvailable()` method to assert whether Kibana
is running on a given pricing product during a plugin server setup
lifecycle step.

##  Why do we need it?

Some feature registrations are synchronous in the plugin setup, and
until all the plugins have registered their features, knowing features
availability at the server setup step is not possible.

## How does this fix it?

To guarantee we have access to all the registered features during the
setup lifecycle, the process works so that:
1. All the plugins are set up and the product features are registered
synchronously.
2. Once all the plugins are set, the core pricing service emits a
signal.
3. The `isFeatureAvailable` method on setup, if invoked previously,
resolves asynchronously, reacting to the emitted signal.

This guarantees access to the product feature model anywhere in Kibana,
without having to rely on the raw tier configuration values.

```ts
public setup(core: CoreSetup) {
    core.pricing.registerProductFeatures([
      {
        id: 'my-plugin:feature1',
        description: 'A feature for observability products',
        products: [
          { name: 'observability', tier: 'complete' },
        ],
      }
    ]);

    core.pricing.isFeatureAvailable('my-plugin:feature1').then((isActiveObservabilityComplete) => {
      if (isActiveObservabilityComplete) {
        // Enable feature1
      }
    });
  }
```

(cherry picked from commit 44c11da)