[go_router_builder] Use a mixin instead of an extension

[StevenSorial]

first step of fixing #106790

This PR changes the generation to use a mixin instead of an extension. The plan is to add the routing methods directly to GoRouteData. An extension won't be able to override the class implementations, so I changed it to a mixin.

I'm not sure about the versioning, so feel free to suggest changes.

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[StevenSorial]

@hannah-hyj

[chunhtai]

LGTM