[firebase_auth] Fix getIdToken refresh param on iOS

[lukaspili]

Description

The function getIdToken() takes a refresh parameter.
This parameter is ignored on iOS, and hardcoded refresh = true is used. This PR fixes that.
No changes required on Android, the refresh parameter is properly taken in account.

Related Issues

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[collinjackson]

Thanks for the PR! This looks challenging to integration test, so I'm going to go ahead and merge it without a test.

[lukaspili]

Thanks for the quick review!