bgpd: Receive SRv6 SIDs notification from zebra

bgpd: Receive SRv6 SIDs notification from zebra

Zebra sends a `SRV6_SID_NOTIFY` notification to inform clients about the
result of a SID alloc/release operation.  This commit adds a handler to
process a `SRV6_SID_NOTIFY` notification received from zebra.

If the notification indicates that a SID allocation operation was
successful, then it stores the allocated SID in the SRv6 database,
installs the SID into the RIB, and advertises the SID to the other BGP
routers.

If the notification indicates that an operation has failed, it logs the
error.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>