[iOS] Add TabGroupPalette colors to the CreateTabGroup view

This CL applies a gradient background to the CreateTabGroupView and adds
specific coloring to the title TextField and the snapshot background.

It also refactors the TabGroupColorPalette class to allow retrieving the
gradient background without instantiating the entire palette. This is
necessary for the CreateTabGroup view, which requires a new gradient
background each time a color button is tapped.

Finally, it adds a new MultiColorGradientView class which supports two
or more colors with custom stop locations. It also adds a
TabGroupGradientView class to handle the gradients in both
CreateTabGroupView and TabGroupView.

screenshot :
https://drive.google.com/file/d/1dyuPCJ2PoHuoE_rl8UoFmYK5Akc9KcJJ/view?usp=sharing&resourcekey=0-GCXy9bNrxJFy1gRqxHo3JQ

record of the animations :
https://drive.google.com/file/d/1hOIkQkrbPoc1c3AgxNzohy89_MH9LiOs/view?usp=sharing&resourcekey=0-UeI-FaN9bfY3ewF_RpwS3Q

Bug: 445887468, 489639989
Change-Id: Ic5f75336c481f397b5af48f7dec647b044609a04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7696472
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Anis Chara <achara@google.com>
Reviewed-by: Ewann Pellé <ewannpv@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1605560}