[EVPN-MH] Add L2 nexthop group table and raw netlink message handling support

[bdfriedman]

Why I did it

This PR adds infrastructure support in sonic-swss-common for EVPN Multi-Homing (EVPN-MH) feature. EVPN-MH requires:

1. A new L2 nexthop group table to manage L2 nexthop groups in APPL_DB
2. Raw netlink message handling capability to process EVPN-MH specific netlink attributes that cannot be fully parsed by libnl's generic object parsing

Work item tracking

• Microsoft ADO (number only):

How I did it

1. Added new APPL_DB table: APP_L2_NEXTHOP_GROUP_TABLE_NAME for managing L2 nexthop groups
2. Implemented raw netlink message handler infrastructure in NetDispatcher:
   • Added registerRawMessageHandler() to register callbacks for raw netlink messages
   • Added unregisterRawMessageHandler() to unregister raw message handlers
   • Added onNetlinkMessageRaw() to handle raw netlink messages (as nlmsghdr*)
   • Added onMsgRaw() virtual method in NetMsg class for derived classes to implement raw message processing
   • Added m_rawhandlers map to store raw message handlers
3. Modified message flow: When a netlink message is not handled by the regular (parsed) handlers, it falls back to raw message handlers

This allows components to choose between:

• Parsed netlink messages (existing behavior via onMsg() with nl_object*)
• Raw netlink messages (new capability via onMsgRaw() with nlmsghdr*)

How to verify it

1. Build sonic-swss-common with these changes
2. Verify that existing netlink message handling continues to work (backward compatibility)
3. Test EVPN-MH feature components that register raw message handlers
4. Verify L2_NEXTHOP_GROUP_TABLE can be accessed from APPL_DB

Description for the changelog

Add support for L2 nexthop group table and raw netlink message handling for EVPN-MH feature

Link to config_db schema for YANG module changes

N/A - This PR only adds APPL_DB table definition, no CONFIG_DB schema changes

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Key Changes:

• Files modified: netdispatcher.cpp, netdispatcher.h, netmsg.h, schema.h
• Total changes: +60 lines, -1 line
• This is infrastructure preparation for EVPN Multi-Homing support in sonic-swss