passkey sample code support in obj-c

[Xiaoshouzi-gh]

This is a copy of pull-request #12128 with rebasing of main branch. All reviewed the code has not been changed except for the following address of comments.

1. Updated the //TODO signInCredential wording.
2. Move the hook of delegator selector logic to MainViewController.m
3. Remove an empty line from FIRUser.m

[google-oss-bot]

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