Add connectivity in NekRSMesh

[aprilnovak]

Reason

I believe the reason why !788 is failing displacement tests for NekRS is because we currently have duplicate nodes shared by elements (and each element is receiving a different displacement from MOOSE).

Design

We should (i) reduce memory usage and (ii) enforce more robust displacement transfers by using connectivity information from NekRS when building the mesh mirror.

Impact

New feature, may need to have a legacy flag to retain previous test behavior.