[google_sign_in] Add future that completes when the plugin is (really) ready. #71607
Description
Use case
Google Sign In (web) is some times slow to start up and silently sign in the current user; however there's no way for app developers to let users know about this delay.
There's currently a future in the plugin side that can be used to compute the initialization status of the plugin, without having to touch platform-dependent code:
(However, it can become null if the initialization fails)
Proposal
As proposed by @SwissCheese5 here:
A Future in the Google Sign In package that completes as soon as the javascript script finishes loading. That way we can show a loading indicator on sites and act accordingly. This will not break anything and only provides an extra feature people can use to make sure google sign in finishes loading.