Saved tab groups: Add auto pin pref boolean

Add a pref boolean to control whether new saved tab groups are pinned
automatically. The value is syncable similar to
bookmark_bar.show_tab_groups and can be toggled under
chrome://settings/appearance

Screencast: https://screencast.googleplex.com/cast/NDg3MTA0Mzk1NjA4MDY0MHxlZjU2NTkxOS0yNw
Bug: 345564560
Change-Id: Ib8852600d42e277197b476d20ea476d238a07e83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5609653


Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Commit-Queue: Yuheng Huang <yuhengh@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: David Pennington <dpenning@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312899}