isisd: Install SRv6 End SID automatically

isisd: Install SRv6 End SID automatically

When zebra assigns a chunk to IS-IS, zebra sends a
ZEBRA_SRV6_MANAGER_GET_LOCATOR_CHUNK notification to IS-IS.
IS-IS invokes the `isis_zebra_process_srv6_locator_chunk()` callback to
process the received notification.

Actually, `isis_zebra_process_srv6_locator_chunk()` iterates over all
areas of the current IS-IS instance and looks for an area for which the
received chunk was requested.
If a match is found, the new chunk is added to the area's chunk list and
`lsp_regenerate_schedule()` is called to regenerate the LSPs to
advertise the new SRv6 locator.

This commit extends the `isis_zebra_process_srv6_locator_chunk()`
function to automatically allocate an SRv6 End SID from the received
chunk and install it in the data plane.

The SRv6 End SID is the instantiation of a Prefix-SID (RFC 8986 section

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