[FastCheckout] Implement FC mechanisms on chrome_autofill_client.

The CL implements FastCheckout related methods on `chrome_autofill_client`.

The feature is available if:
* The feature flag is on and FC is available on the form.
* User has autofill profiles and cc enabled.
* Autofill assistant flag is on.

We start FC if:
* no other autofill assistant flow is running.

Bug: 1334642
Change-Id: If64db4b0ac998d357d52d59d6cdc45aecc630e49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3846091


Reviewed-by: Jan Keitel <jkeitel@google.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Norge Vizcay <vizcay@google.com>
Cr-Commit-Position: refs/heads/main@{#1041596}