[fuchsia] Mark conditionally-used capabilities as optional

web_engine_shell.cml uses many protocols for the sake of having them in
its svc directory, which it passes off to web_instance. In this CL, we
mark those that are conditionally used based on buildtime or runtime
configuration as `optional`.

Bug: 1352062, b/264228941
Change-Id: Ib5ef8c28d0e1b5ec5b022a2a1bba24f278311472
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4161733
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1091789}