[ SavedTabGroup ] Add configurable FeatureParam for nudge delay

Adds a FeatureParam that will be used to experiment with different nudge
delay times for the SavedTabGroups feature.

Bug: 1448892
Change-Id: I615abd36eaf542485083fd7ed33bc8a5f89e8255
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4559065


Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Darryl James <dljames@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1149234}