feat: add cached VLAN and LAG Redis collectors

[rokernel]

Summary

• add a new VLAN collector that reads VLAN|*, VLAN_MEMBER|*, and VLAN_TABLE:* from Redis and exports VLAN/member state metrics
• add a new LAG collector that reads PORTCHANNEL|*, LAG_TABLE:*, and LAG_MEMBER_TABLE:* from Redis and exports PortChannel/member state metrics
• keep scrape path safe by using background refresh + cached metric snapshots for both collectors (same protection model as LLDP)
• register new collectors in the main exporter and add environment-based guardrails for refresh interval, timeout, and max items
• extend fixtures and tests to validate VLAN and LAG metric output

Why

You requested VLAN/PortChannel visibility from SONiC Redis with low impact on switch CPU and scrape latency. This design keeps the exporter read-only and prevents LLDP/VLAN/LAG refresh work from blocking /metrics scrapes.

Verification

• go test ./...
• go build ./...
• ./scripts/build.sh

[rokernel]

Reopening to retrigger pull_request checks.