isisd: Add data structure for SRv6 Adjacency SIDs

isisd: Add data structure 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 a data structure `srv6_adjacency` for storing
information about an SRv6 Adjacency SID. This data structure will be
used to support SRv6 Adjacency SIDs functionalities in future commits.

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