RPKI cache-connection status metrics

[tynany]

Implement #156

New collector: collector/rpki.go

• Disabled by default, enabled with --collector.rpki
• Automatically queries all VRFs by reusing the existing getVRFs() helper
• Runs show rpki cache-connection json for the default VRF and show rpki cache-connection vrf json for each additional VRF
• Exposes two metrics:
  • frr_rpki_cache_state: 1 = connected, 0 = disconnected
  • frr_rpki_cache_preference: preference value
• Labels: vrf, mode, host, port

Tests: collector/rpki_test.go

• Tests processing of default and VRF-specific JSON output
• Two test fixtures in collector/testdata/

Refactor: moved getVRFs() / parseVRFs() from route.go to collector.go

• Now shared between the route and RPKI collectors

Dev environment (dev/)
RPKI is not supported on the default FRR container, so this is skipped. May consider compiling a custom version in the future.