Add SAI L2 bridging, FDB learning, and BUM flooding support

[thineshvs-google]

Add SAI L2 bridging, FDB learning, and BUM flooding support in Lemming

This change implements native Layer 2 Ethernet switching and MAC address learning in Lemming's standalone forwarding pipeline:

1. Dataplane Tables & Pipeline (switch.go, ports.go):

   • Initializes FDBTable ("fdb") for exact-match MAC forwarding.
   • Initializes FloodTable ("flood") keyed on VLAN_TAG for BUM flooding.
   • Integrates BridgeLearnAction(FDBTable) and LookupAction(FDBTable) into ingress port pipelines.
   • Falls back to LookupAction(FloodTable) on unicast lookup miss or broadcast traffic.

2. SAI FDB Handling (fdb.go):

   • Adds CreateFdbEntry and RemoveFdbEntry handlers.
   • Maps learned MAC and VLAN IDs directly to port transmit actions in FDBTable.

3. VLAN Flood Groups (routing.go):

   • Creates aggregate flood groups ("vlan_flood_") in CreateVlan.
   • Dynamically adds/removes port members in CreateVlanMember and RemoveVlanMember with InstanceCount: 1 to ensure active aggregate member forwarding.

4. Bug Fixes (ports.go, routing.go):

   • Fixes 16-bit VLAN_TAG big-endian byte encoding in exact-match table lookups and port updates.