bgpd, lib, zebra: Implement handling of BGP-Prefix-SID label Index

bgpd, lib, zebra: Implement handling of BGP-Prefix-SID label Index

Implement BGP Prefix-SID IETF draft to be able to signal a labeled-unicast
prefix with a label index (segment ID). This makes it easier to deploy
global MPLS labels with BGP, even without other aspects of Segment Routing
implemented.

This patch implements the handling of the BGP-Prefix-SID Label Index
attribute. When received from a peer and the index is acceptable, the local
label is picked up from the SRGB and is programmed as the incoming label as
well as advertised to peers. If the index is not acceptable, no local label
is assigned. The outgoing label will always be the one advertised by the
downstream neighbor.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>