Need external ICU callbacks for Chromium integration

Chromium updates ICU project wide and existing C-HarfBuzz is configured with `HAVE_ICU` and depends on `"//third_party/icu:icuuc"`.

This enables us to control ICU behavior in shaping in sync with other ICU behavior of Chromium.

If we only rely on built-in ICU and the `scripts/update-unicode-tables.mk` script, we would have overhead for keeping HarfRust and rest of Chromium in sync on ICU version. This becomes particularly tricky, should we need to roll back ICU when upgrading to a new version.