feat: add support for custom router state serializer

[Copilot]

This PR adds support for custom router state serializers to both provideGlobalRouterStore and provideLocalRouterStore, bringing feature parity with @ngrx/router-store.

Problem

As noted in the comparison documentation, Router Component Store previously used a serializer similar to MinimalRouterStateSerializer but did not support custom router state serializers, unlike NgRx Router Store which supports this through a serializer setting.

Solution

This implementation adds:

Core API

• Generic RouterStateSerializer<T> interface - Allows custom serializers with type safety
• ROUTER_STATE_SERIALIZER injection token - Enables dependency injection of custom serializers
• RouterStoreConfig<T> interface - Configuration object for provider functions

Provider Updates

Both provider functions now accept an optional configuration parameter:

// Global router store with custom serializer
providers: [
  provideGlobalRouterStore({ serializer: CustomRouterStateSerializer })
]

// Local router store with custom serializer  
@Component({
  providers: [provideLocalRouterStore({ serializer: CustomRouterStateSerializer })]
})
export class MyComponent { }

Example Custom Serializer

@Injectable()
export class CustomRouterStateSerializer implements RouterStateSerializer<MinimalRouterStateSnapshot> {
  serialize(routerState: RouterStateSnapshot): MinimalRouterStateSnapshot {
    return {
      root: this.serializeRoute(routerState.root),
      url: '/custom' + routerState.url, // Custom URL transformation
    };
  }

  private serializeRoute(route: ActivatedRouteSnapshot): MinimalActivatedRouteSnapshot {
    // Custom serialization logic
  }
}

Backward Compatibility

✅ Zero breaking changes - All existing code continues to work unchanged. When no custom serializer is provided, the default MinimalRouterStateSerializer is used automatically.

Use Cases

This feature enables:

• URL Transformation: Modify URLs before storage (e.g., adding prefixes, normalization)
• Data Enrichment: Add timestamps, user context, or metadata to router state
• Filtering: Remove sensitive information from router state
• Custom State Structures: Create specialized state formats for specific needs

Testing

• Added comprehensive test suite with 7 new test cases covering all scenarios
• All existing tests continue to pass (264 total tests)
• Includes tests for both global and local router stores with custom serializers
• Verifies default behavior when no custom serializer is provided

Documentation

• Updated README with detailed examples and usage patterns
• Updated comparison documentation to reflect new feature parity with NgRx Router Store
• Added important notes about type safety and compatibility requirements

This brings Router Component Store to full feature parity with @ngrx/router-store regarding custom serialization while maintaining the lightweight, type-safe approach that makes it an excellent alternative to both NgRx Router Store and ActivatedRoute.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add support for custom serializer and custom router state type</issue_title>
<issue_description>The current ngrx router-store supports provofing a custom state serializer and working with a custom state type. router-component-store should support this too.</issue_description>

Comments on the Issue (you are @copilot in this section)

@LayZeeDK Thank you for your feature request. What's a use case for this? @LayZeeDK I'm not very familiar with `@ngrx/router-store` but from what I can read, a router state serializer is used to extract more slices of state from `RouterStateSnapshot`, correct? @LayZeeDK I'm not sure how adding this is better than subscribing to the `RouterState` through the `Router` and mapping using observable operators. That seems to be a lot easier than implementing a router serializer.

In NgRx RouterStore this functionality was added to support Redux Dev Tools. Currently, NgRx ComponentStore doesn't have native support for Redux Dev Tools.</comment_new>
<comment_new>@LayZeeDK
Even if we were to add it, I worry it would increase the bundle size significantly even for the xx% of applications never using this feature.</comment_new>

Fixes #260

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