[fuchsia] Launch web_instance.cm from context_provider.cm

In so doing:

- Add support for CreateContextParams.service_directory in
  WebInstanceHost.
- Add support to WebInstanceHost for running within a process with
  `--with-webui` on its command line to launch child instances in the
  `web_engine_with_webui` package rather than the `web_engine`
  package. This feature is used by `web_engine_shell` in test scenarios.

Bug: 1280703
Fuchsia-Binary-Size: Size increase is unavoidable.
Change-Id: I08d65492c4c670e67cca88beb223d1d8c8eed76f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4194689


Reviewed-by: Rohan Pavone <rohpavone@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1108989}