[fdomain_local] Add local_client_fdomain

This lets you create a local FDomain and populate the namespace of that
FDomain using only FDomain handles. Previously to have your local
FDomain have a namespace you had to use real or emulated/Overnet
handles.

This enables host-side tests to not depend on Overnet or emulated
handles.

Change-Id: Ic1a0244d0d2f823c9298a99811ca347e6a6a6964
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1783278
Reviewed-by: Steven Grady <slgrady@google.com>
Commit-Queue: Casey Dahlin <sadmac@google.com>
Fuchsia-Auto-Submit: Casey Dahlin <sadmac@google.com>