[go_router]: add optional completer for replaced routes

[pawlowskim]

When route from the stack is replaced, its completer is lost, and causes futures in routes chain to never finish. This change add optional completer that can be passed when pushing / replacing routes that will be completed when route is replaced.

It fixes: https://github.com/flutter/flutter/issues/141251 This PR is continuation of: https://github.com/flutter/packages/pull/6451

CC @chunhtai

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