isisd: Receive SRv6 SIDs notifications from zebra

isisd: Receive SRv6 SIDs notifications 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 IS-IS
routers.

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

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