isisd: Add function to process received SRv6 chunk

isisd: Add function to process received SRv6 chunk

Add a callback function that is called upon receiving an SRv6 locator
chunk from zebra.

This function 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.
If no match is found, we free the allocated resources and do nothing.

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