bgpd: Add `neighbor PEER link-bw-encoding-ieee`

bgpd: Add `neighbor PEER link-bw-encoding-ieee`

This is to avoid breaking changes between existing deployments of
extended community for bandwidth encoding. By default FRR uses uint32
to encode bandwidth, which is not as the draft requires (IEEE floating-point).

This switch enables the required encoding per-peer.

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>