isisd: Add func to process a received SRv6 locator

isisd: Add func to process a received SRv6 locator

Add a callback function `isis_zebra_process_srv6_locator_add()` that is
called upon receiving an SRv6 locator from zebra.

When a new SRv6 locator is created in zebra, zebra sends a
ZEBRA_SRV6_LOCATOR_ADD notification to all daemons informing them of the
new locator.

In IS-IS, we register the new `isis_zebra_process_srv6_locator_add()`
callback as the handler for ZEBRA_SRV6_LOCATOR_ADD.

This callback iterates over all areas of the current IS-IS instance and
looks for an area for which the new locator was configured.
If a match is found, we call
`isis_zebra_srv6_manager_get_locator_chunk()` to ask zebra a chunk from
the locator.
If no match is found, we do nothing.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>