[go_router] Support passing extra object alongside `initialLocation` while creating GoRouter

## Use case
Currently only `initialLocation` can be passed while creating a **GoRouter** instance. There is no way to pass extra object.
This should mostly be useful in scenario when there are nested routers or in tests.

## Proposal
A new param can be added to **GoRouter**, named `initialExtra`.

```dart
GoRouter(
  initialLocation: '/dummy',
  initialExtra: ExtraDataForDummy(),
)
```
