[go_router] ShellRoute will merge GoRouter's observers

[huanghui1998hhh]

ShellRoute currently does not trigger observers defined in GoRouter, which is unexpected behavior.

Fixes flutter/flutter#112196

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[chunhtai]

why make this a per route setting instead of a global setting in goRouter?

[chunhtai]

also what if it is a shellroute nested inside another shell route, if this is true, should the innershellroute notify the observer of outer shellroute?

[huanghui1998hhh]

Maybe we should use Navigator.maybeOf(context).widget.observers to merge observers and keep the mergeObservers property for each ShellRoute.

This ensures that the case of ShellRoute nesting ShellRoute is handled correctly, but there is one exception: when ShellRoute is nested under a Navigator that is not built by ShellRoute. Maybe we should ignore this case.

The reason for adding mergeObservers property to each ShellRoute is to allow users to decide that a specific ShellRoute should not merge observers. Defining it as a global setting in GoRouter would make maintaining this case difficult.