[session] Add shard for NamedKeyProvider

This creates the file and core shard, we will add contents in a
future change.

Bug: 124328
Change-Id: I82838b28605f82d90dcc5522e21af1761580cf8b
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/826576
Reviewed-by: Yegor Pomortsev <ypomortsev@google.com>
Commit-Queue: Justin Mattson <jmatt@google.com>