[go_router]: Add RelativeGoRouteData and TypedRelativeGoRoute

[LukasMirbt]

Adds RelativeGoRouteData and TypedRelativeGoRoute to support type-safe relative routes.

Foundation for #9749 which adds relative route support in go_router_builder.

Continuation of #8476 by @ThangVuNguyenViet

Compared to #8476, the approach taken in this PR avoids breaking changes and creates a separate RouteData subclass instead of extending GoRouteData. The approach increases flexibility and avoids extending behavior that might not make sense for a relative route.

Necessary to fully resolve flutter/flutter#108177.

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Code Review

This pull request introduces type-safe relative routes by adding RelativeGoRouteData and TypedRelativeGoRoute. The changes are well-tested. My main feedback concerns significant code duplication in the new RelativeGoRouteData class, which poses a maintainability risk. I've also pointed out a minor bug and some areas for improvement in the new tests.

[bbhawal-atec]

I am curious how this would work with deeplinking, or whether it would even be possible to support it

[LukasMirbt]

I am curious how this would work with deeplinking, or whether it would even be possible to support it

Deep-linking should be fully supported!

If you have a use case you want to test before this PR is merged, add the following to your pubspec.yaml:

dependencies:
  go_router:
    git:
      url: https://github.com/LukasMirbt/packages.git
      ref: relative-routes
      path: packages/go_router

dev_dependencies:
  go_router_builder:
    git:
      url: https://github.com/LukasMirbt/packages.git
      ref: relative-routes
      path: packages/go_router_builder

[hannah-hyj]

LGTM

[LukasMirbt]

@chunhtai Can this PR along with #9749 be auto-submitted or what's the next step?