BFD support for BGP

[rcgoodfellow]

BFD support for BGP similar to Cisco, Juniper and others provide.

[taspelund]

For context to help with prioritization:

BFD is its own protocol which establishes and manages adjacencies independently of routing protocols.
However, routing protocol adjacencies are often linked to the operational state of a BFD adjacency.
This enables routing protocols to piggy-back on the sub-second failure detection that BFD provides.

This ticket is essentially tracking a new feature: to update maghemite and related components such that BFD sessions can be linked to BGP sessions and convergence can be triggered upon a BFD failure.

Many networks enable BFD for their routing protocol adjacencies as a standard config and it may come as an assumed feature when a user sees that Oxide supports BFD.