[go_router_builder] Add support for relative routes

[ThangVuNguyenViet]

This PR is a 2nd part of #6825 to fully resolves flutter/flutter#108177, which allow users to use TypedRelativeGoRoute to construct the route relatively.

This is a continuation of #7174

Example:

import 'package:go_router/go_router.dart';

const TypedRelativeGoRoute<RelativeRoute> relativeRoute =
    TypedRelativeGoRoute<RelativeRoute>(
  path: 'relative-route',
  routes: <TypedRoute<RouteData>>[
    TypedRelativeGoRoute<InnerRelativeRoute>(path: 'inner-relative-route')
  ],
);

@TypedGoRoute<Route1>(
  path: 'route-1',
  routes: <TypedRoute<RouteData>>[relativeRoute],
)
class Route1 extends GoRouteData {
  const Route1();
}

@TypedGoRoute<Route2>(
  path: 'route-2',
  routes: <TypedRoute<RouteData>>[relativeRoute],
)
class Route2 extends GoRouteData {
  const Route2();
}

class RelativeRoute extends GoRouteData {
  const RelativeRoute();
}

class InnerRelativeRoute extends GoRouteData {
  const InnerRelativeRoute();
}

Basically the added TypedRelativeGoRoute allows us to construct the route tree above. If we replace it with the existing TypedGoRoute it will declare multiple extensions of a same thing and produce build time error

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[ThangVuNguyenViet]

@chunhtai same issue I asked in the old PR. The test will fail without the temp dependency_overrides. Should I make a PR that contains only the TypedRelativeGoRoute in go_router and wait for it to get merged first?