ARISTA - BGP sessions under VRF

Hi @gibbsj87
I would suggest to start with this PR.

The code is improving BGP support for huawei VPR devices, using their MIB instead of the standard BGP4 (which does not support BGP sessions under VRFs).

This code will be merged (1st step) and a 2nd step (refactoring) will come to split this code into different files (one per OS). But you can already start working/testing on Arista devices, and when the 2 PR are merged, we’ll add your Arista code.