[WebUI] Add trusted_types_util to the Ash WebUI common directory

This CL adds a trusted_types_util library to the `//ash/webui/common`
directory that provides Trusted-Types related utility functions. It
also adds a `ash::EnableTrustedTypesCSP` function that mirrors the
behaviors of `webui::EnableTrustedTypesCSP` living under chrome.

Bug: 1449822
Change-Id: I3a46166891714f32b54ac54484a7610266da0475
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4573172
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Commit-Queue: Junguo (Jun) W. <jungoog@google.com>
Cr-Commit-Position: refs/heads/main@{#1157309}