[reachability] Log ping/fetch results to Inspect

Fixed: 492604858
Bug: 492604663
Test:
1. Added unit tests
2. Verified `ffx inspect show core/network/reachability`

Change-Id: I40ec21b881260b8aef744b04d558d1bd86bc6fef
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1535340
Reviewed-by: Antonio Linhart <antoniolinhart@google.com>
Commit-Queue: Kiet Tran <kiettran@google.com>
6 files changed