isisd: Add SRv6 MSDs to Router Capabilities TLV

isisd: Add SRv6 MSDs to Router Capabilities TLV

Add support for SRv6 Node MSDs as per RFC 9352 section #4.

There are four types of SRv6 MSDs:
* Maximum Segments Left MSD Type
* Maximum End Pop MSD Type
* Maximum H.Encaps MSD Type
* Maximum End D MSD Type

These SRv6 Node MSDs are advertised in the Node MSD Sub-TLV, a Sub-TLV
of the Router Capabilities TLV.

This commit adds `struct isis_srv6_msd` data structure, which represents
the SRv6 MSDs advertised in the Node Sub-TLV.

This commit also adds the `struct isis_srv6_msd` to
`struct isis_router_cap`, a data structure representing the Router
Capabilities TLV.

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