[IOS] Add feature flag for the "Stay signed out" FRE sign in promo

This CL introduces a feature flag, kStaySignedOutFrePromo, to
control the display of the "Stay Signed Out" text in the FRE
sign-in promo.

Design Doc: https://docs.google.com/document/d/1mUbfDCjyd2ppr0qoljVfXXbMaoSxQ97jDYlM51XkSc0

Bug: 385157439

Change-Id: I7f9495e4b1d2d979b6f1acf7bc8ca5ae62f2a820
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6108399
Reviewed-by: Cheick Cisse <cheickcisse@google.com>
Reviewed-by: Guillaume Jenkins <gujen@google.com>
Commit-Queue: Ibrahim Kanouche <kanouche@google.com>
Cr-Commit-Position: refs/heads/main@{#1398773}