Factor extensions out of GetAuthenticatorAssertionResponse.

Before this change extension properties were flattened into
GetAuthenticatorAssertionResponse, now
AuthenticationExtensionsClientOutputs holds these extension properties.
All references (read/write) are updated to be accessed through the new
extensions property on GetAuthenticatorAssertionResponse, and the new
extensions property is initialized at each site where
GetAuthenticatorAssertionResponse is itself initialized.

This change will enable reusing conversion code where extensions are
as part of another type such as SecurePaymentConfirmationResponse
(in an upcoming change).

https://docs.google.com/document/d/1UMK2ri8Wmd34WpRmXoWBycmRHx7N6DmbUkktsBVOVE8/edit?usp=sharing

Bug: 1365339
Change-Id: I43700204654f47c321945d4174c03ab1d38d3bf5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4863119
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Auto-Submit: Slobodan Pejic <slobodan@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1202215}