mgmtd/zebra: zebra: add per-source route-count support under /frr-zebra:lib/vrf

[ashred-lnx]

• Add zebra per-source route-count operational support under /frr-zebra:lib/vrf (IPv4/IPv6 + total).
• Expose counts through YANG/NB callbacks backed by zebra RIB per-route-type counters.
• Keep selector path as /frr-zebra:lib/vrf/... for client compatibility.
• Add generic periodic FE sampling topotest coverage including the new zebra route-count selector.

Testing:

`sudo ./vtysh/vtysh -c 'show mgmt get-data /frr-zebra:lib/vrf/ipv4-route-count/total datastore operational json'
timeout 15s sudo python3 tests/topotests/lib/fe_client.py \
  --server /var/run/frr/mgmtd_fe.sock \
  --listen "/frr-zebra:lib/vrf/ipv4-route-count/total" \
  --notify-mode periodic \
  --notify-mode-data 1000 \
  --notify-count 5 \
  --datastore \
  -v
{"frr-zebra:lib":{"vrf":[{"id":"default","ipv4-route-count":{"total":3}}]}}
2026-03-20 21:59:24,822 DEBUG: Connecting to server on /var/run/frr/mgmtd_fe.sock
2026-03-20 21:59:24,822 INFO: Connected to server on /var/run/frr/mgmtd_fe.sock
2026-03-20 21:59:24,822 DEBUG: Sent native SESSION-REQ
2026-03-20 21:59:24,822 DEBUG: Recv native SESSION-REPLY Message: sess-id 1003: fixed: (1,): b''
2026-03-20 21:59:24,822 DEBUG: Sent NOTIFY_SELECT mode=1 mode_data=1000
2026-03-20 21:59:24,822 DEBUG: Waiting for Notify Message
2026-03-20 21:59:24,833 DEBUG: Received Notify Message: (2, 4): b'/frr-zebra:lib/vrf/ipv4-route-count/total\x00{"frr-zebra:lib":{"vrf":[{"id":"default","ipv4-route-count":{"total":3}}]}}\x00'
#OP=SYNC: /frr-zebra:lib/vrf/ipv4-route-count/total
{"frr-zebra:lib":{"vrf":[{"id":"default","ipv4-route-count":{"total":3}}]}}
2026-03-20 21:59:24,833 DEBUG: Waiting for Notify Message
2026-03-20 21:59:25,835 DEBUG: Received Notify Message: (2, 4): b'/frr-zebra:lib/vrf/ipv4-route-count/total\x00{"frr-zebra:lib":{"vrf":[{"id":"default","ipv4-route-count":{"total":3}}]}}\x00'
#OP=SYNC: /frr-zebra:lib/vrf/ipv4-route-count/total
{"frr-zebra:lib":{"vrf":[{"id":"default","ipv4-route-count":{"total":3}}]}}
2026-03-20 21:59:25,835 DEBUG: Waiting for Notify Message
2026-03-20 21:59:26,836 DEBUG: Received Notify Message: (2, 4): b'/frr-zebra:lib/vrf/ipv4-route-count/total\x00{"frr-zebra:lib":{"vrf":[{"id":"default","ipv4-route-count":{"total":3}}]}}\x00'
#OP=SYNC: /frr-zebra:lib/vrf/ipv4-route-count/total
{"frr-zebra:lib":{"vrf":[{"id":"default","ipv4-route-count":{"total":3}}]}}
2026-03-20 21:59:26,836 DEBUG: Waiting for Notify Message
^Z
`