isisd: Add format function for SRv6 Locator TLV

isisd: Add format function for SRv6 Locator TLV

Add a function to return information about an SRv6 Locator TLV and all
its Sub-TLVs (RFC 9352 section #7.1).

Examples:

r1# show isis database detail
Area 1:
IS-IS Level-1 link-state database:
LSP ID                  PduLen  SeqNumber   Chksum  Holdtime  ATT/P/OL
r1.00-00             *    248   0x00000005  0xda6e     824    0/0/0
[...]
  SRv6 Locator: fc00:0:1::/48 (Metric: 0) standard
    Sub-TLVs:
[...]

r1# show isis database detail json
[...]
              "srv6-locator":{
                "mt-id":0,
                "prefix":"fc00:0:1::/48",
                "metric":0,
                "d-flag":"",
                "algorithm":0,
                "mt-name":"standard",
                "subtlvs":{

                }
              }
[...]

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