isisd: Add SRv6 SID data structure

isisd: Add SRv6 SID data structure

Add `struct isis_srv6_sid` data structure to represent an SRv6 SID.

The struct has the following members:
* next: pointer to the next SID, used to build linked lists of SRv6 SIDs
* flags: SID flags
* behavior: the behavior bound to the SRv6 SID (as defined in RFC 8986)
* value: the SID value (i.e., an IPv6 address)
* locator: a backpointer to the parent locator from which the SID has
been allocated.

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