[fuchsia] Ensure services are not unintentionally offered from host component

Previously, CreateInstanceForContextWithCopiedArgs() would offer
services from the current Component, the `web_instance` host, iff no
`service_directory` was provided in the provided `CreateContextParams`.
This could lead to mistakes, like the one fixed by
https://crrev.com/c/4528805,  or even abuse if no `service_directory`
or a partial `service_directory` was provided. Since the two modes are
for very different use cases, split them out into separate
sub-classes.

Bug: 1280703, 1421351
Change-Id: Ia2d3f71209b42f84d27d632de91e3a048023b237
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4358796
Auto-Submit: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1143718}