isisd: Add support for SRv6 Adjacency SIDs

isisd: Add support for SRv6 Adjacency SIDs

An SRv6 adjacency SID is a SID that is associated with a particular
adjacency. Adjacency SIDs are advertised using the SRv6 End.X SID
Sub-TLV (RFC 9352 section #8.1) or SRv6 LAN End.X SID Sub-TLV (RFC 9352
section #8.2).

This commit defines the following Adj SIDs management functions:
* srv6_endx_sid_add_single: add a new SRv6 Adjacency SID
* srv6_endx_sid_del: delete an SRv6 Adjacency SID
* isis_srv6_endx_sid_find: lookup SRv6 End.X SID by type

It also attaches some callbacks to the hooks isis_adj_state_change_hook,
isis_adj_ip_enabled_hook, isis_adj_ip_disabled_hook, which are
responsible for installing/removing an SRv6 Adjacency SID automatically
when the state of an IS-IS adjacency changes.

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