isisd: Add CLI command to show SRv6 node info

isisd: Add CLI command to show SRv6 node info

Add a CLI command to print SRv6 capabilities, algorithms and MSDs
supported by the IS-IS nodes.

Example:

r1# show isis segment-routing srv6 node
Area FOO:
 IS-IS L1 SRv6-Nodes:

 IS-IS L2 SRv6-Nodes:

 System ID       Algorithm  SRH Max SL  SRH Max End Pop  SRH Max H.encaps  SRH Max End D
 -----------------------------------------------------------------------------------------
 1111.1111.1111  SPF        16          0                1                 2
 2222.2222.2222  SPF        16          0                1                 2

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