[fuchsia] Add support for using FIDL natural bindings

Most of the new code is copied from the existing HLCPP versions,
but unfortunately git doesn't track copies, so they all show up
as new. Once migration is complete, the hlcpp variants can be
removed and the "natural" part of the names in this CL can also
be removed.

See go/natural-component-context (googlers only) for more context on some of the choices made.

Bug: 1351487
Change-Id: I1c94398a3f05d842c97c83ea4a1b1326e9ef2e98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4229073


Commit-Queue: Bryant Chandler <bryantchandler@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1112870}