[iOS]Adds the completion while creating the signin coordinator

While creating the SigninCoordinator from the ShowSigninCommand, it was
expected all properties to be used. In crrev.com/c/6516183, the
SigninCompletion was missing.

It was not an issue because the completion was put by the caller, but
given the function name and documentation, it should use all of the
command’s properties.

This CL don’t change the SceneController because actually, in this
particular case, the completion to be executed is not the one from the
ShowSigninCommand, so the SceneController still have to reassign the
completion after the SigninCoordinator is created.

Bug: 381444097
Change-Id: I8d02b2c12cbea568d8fa009180450acd0db6cdc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6543490
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Auto-Submit: Arthur Milchior <arthurmilchior@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460136}