TLDR; publish draft enhancement for EVPN

There are further discussions required with upstream stakeholders to refine the current upstream enhancement however, we can create a draft downstream enhancement to record what we will attempt and it will be a place to refine our overall goal and crucially get input from many stakeholders downstream on what said parties would like.

Lets try to get a draft document ready for sprint 277, hence why this is a spike and not a story.

Anything in active discussions can be ommitted and stated what the discussions currently are. This will allow us to track upstream discussions and design choices on the downstream enhancement PR.