[go_router] Add route metadata support to GoRouterState.

[star4277]

This PR adds route metadata support across go_router route definitions, match resolution, and GoRouterState, including inheritance/override behavior and test coverage.

• Added metadata support to RouteBase and all relevant route types.
• Propagated merged route metadata into GoRouterState.
• Added metadata resolution/merge logic in route matching:
• inherit from parents
• child keys override parent keys
• default to empty map when absent
• Wired top-level/error state builders to include metadata.
• Added regression test coverage for metadata inheritance/override/empty behavior.

Fixes flutter/flutter#187049, flutter/flutter#160738.

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Code Review

This pull request introduces a metadata property to RouteBase and GoRouterState, enabling the association of custom data with routes. The implementation includes logic within RouteMatchList to handle metadata inheritance and overriding during route matching. GoRouterState is updated to incorporate this metadata into its equality and hashing logic, and new tests verify the expected behavior. I have no feedback to provide.

[stuartmorgan-g]

[x] I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under1.

@star4277 I don't see this reflected in the PR. Are there changes you forgot to push?

[star4277]

[x] I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under1.

@star4277 I don't see this reflected in the PR. Are there changes you forgot to push?

Sorry, I missed it, I've made it up now