[core] Add a diagnostics sub-realm

Creates a /core/diagnostics sub-realm and moves
diagnostics-persistence and detect into it.

Bug: 120916

Change-Id: I4728b6ef2d0f2e2e848959ecb216856a82e9ef3f
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/788612
Reviewed-by: Aaron Wood <aaronwood@google.com>
Commit-Queue: Justin Mattson <jmatt@google.com>
Reviewed-by: David Iserovich <diserovich@google.com>
Reviewed-by: Miguel Flores <miguelfrde@google.com>