feat: Add support for hooks.

[kinyoklion]

Add support for hooks to the SDK.

Example of a hook with all methods that logs things.

final class TestHook extends Hook {
  final HookMetadata _metadata = const HookMetadata(name: "test-hook");

  @override
  HookMetadata get metadata => _metadata;

  @override
  UnmodifiableMapView<String, LDValue> beforeEvaluation(
      EvaluationSeriesContext hookContext,
      UnmodifiableMapView<String, LDValue> data) {
    print(
        'TestHook.beforeEvaluation called with: hookContext=$hookContext, data=$data');
    return super.beforeEvaluation(hookContext, data);
  }

  @override
  UnmodifiableMapView<String, LDValue> afterEvaluation(
      EvaluationSeriesContext hookContext,
      UnmodifiableMapView<String, LDValue> data,
      LDEvaluationDetail<LDValue> detail) {
    print(
        'TestHook.afterEvaluation called with: hookContext=$hookContext, data=$data, detail=$detail');
    return super.afterEvaluation(hookContext, data, detail);
  }

  @override
  UnmodifiableMapView<String, LDValue> beforeIdentify(
      IdentifySeriesContext hookContext,
      UnmodifiableMapView<String, LDValue> data) {
    print(
        'TestHook.beforeIdentify called with: hookContext=$hookContext, data=$data');
    return super.beforeIdentify(hookContext, data);
  }

  @override
  UnmodifiableMapView<String, LDValue> afterIdentify(
      IdentifySeriesContext hookContext,
      UnmodifiableMapView<String, LDValue> data,
      IdentifyResult result) {
    print(
        'TestHook.afterIdentify called with: hookContext=$hookContext, data=$data, result=$result');
    return super.afterIdentify(hookContext, data, result);
  }

  @override
  void afterTrack(TrackSeriesContext hookContext) {
    print('TestHook.afterTrack called with: hookContext=$hookContext');
    super.afterTrack(hookContext);
  }
}

Minimal hook:

Example of a hook with all methods that logs things.
```dart
final class TestHook extends Hook {
  final HookMetadata _metadata = const HookMetadata(name: "test-hook");

  @override
  HookMetadata get metadata => _metadata;

[kinyoklion]

bugbot review

[kinyoklion]

bugbot review

[kinyoklion]

Updating these tests for environment ID now that it has been added.

[kinyoklion]

This method didn't need to be async as it runs only on already resolved promises.