[local_auth] implement stopAuthentication for ios

[masterromuald]

This PR implements flutter/flutter#166413

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[Piinks]

Hey @masterromuald, would you like to continue working on this change? If so, marking it ready for review (it is currently a draft) will put it in the review queue. Thanks!

[stuartmorgan-g]

Since this is marked as a draft and hasn't been updated in several months, I’m going to close it so that our PR queue reflects active PRs. Please don't hesitate to submit a new PR if you decide to revisit this. Thanks!