isisd: Add nb command to configure an SRv6 locator

isisd: Add nb command to configure an SRv6 locator

Add a northbound command to configure an SRv6 locator for a specific
IS-IS area.

After configuring a locator, `isis_zebra_srv6_manager_get_locator_chunk`
is called to ask zebra to allocate a chunk from the configured locator.
The allocated chunk will be owned by IS-IS. IS-IS can allocate SIDs from
its chunk.

Currently, we support only one locator per-area. Therefore, before
configuring a locator we unset the previously configured locator, if
there was any.

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