feat: add SNMP provider for receiving traps as alerts

[vasquezmyguy-collab]

Summary

Implements SNMP provider for receiving SNMP traps as Keep alerts (#2112).

Changes

• SnmpProvider class following the consumer pattern (like Kafka provider)
• Supports SNMP v1, v2c, and v3 (noAuthNoPriv / authNoPriv / authPriv)
• Converts SNMP trap var-binds to Keep alert format with severity mapping
  • linkDown → high severity
  • authenticationFailure → warning
  • coldStart, warmStart, linkUp → info
• Uses pysnmp-lextudio v6 asyncio API
• Includes mock alert data for linkDown and coldStart traps
• Adds pysnmp-lextudio dependency

How it works

The provider listens on a configurable UDP port (default: 162) for incoming SNMP traps. When a trap is received, the NotificationReceiver callback converts var-binds to a Keep alert dict and pushes it via _push_alert().

Testing

• Syntax validated
• Provider follows existing patterns (Kafka consumer provider as template)
• pysnmp v6 asyncio API verified working with open_server_mode + add_transport + open_dispatcher

/claim #2112

Closes #2112

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