[iOS] Add SigninCompletionInfo to ShowSigninCommandCompletionCallback

This CL replaces
```
using ShowSigninCommandCompletionCallback =
    void (^)(SigninCoordinatorResult result);
```
with
```
using ShowSigninCommandCompletionCallback =
    void (^)(SigninCoordinatorResult result, SigninCompletionInfo*);
```
so that the AddAccountSigninCoordinator can be used with a
ShowSigninCommand. Without this change, it is not possible to know what
identity was added since it is contained in the SigninCompletionInfo
provided by AddAccountSigninCoordinator.

Fixed: 1473294
Change-Id: I2d25aec142968f613c671bcd1c81757605cc2cbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4783807
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Cr-Commit-Position: refs/heads/main@{#1184190}