Allow to determine IPv6 routing strategy

Summary:
The method network_utils.can_directly_be_discovered can be invoked
to determine if we can directly reach the gateway (HE for example)
or if we need an intermediate route to that gateway (OVH for example).

Ref T1853

Test Plan: Unit tests provided for new Python functions

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1853

Differential Revision: https://devcentral.nasqueron.org/D3077