Implements SolarWinds provider support for Keep via webhook ingestion.

/claim #3526 Closes #3526

What’s included

• Added new provider: keep/providers/solarwinds_provider/solarwinds_provider.py
• Added webhook payload parsing for common SolarWinds fields:
  • AlertObjectID, AlertName, Severity, NodeName, EntityCaption, Message
  • IsAcknowledged, IsActive, TriggeredDateTime, AlertDetailsUrl
• Added severity mapping:
  • Information/Info -> INFO
  • Warning/Minor -> WARNING
  • Major -> HIGH
  • Critical -> CRITICAL
• Added status mapping logic:
  • IsAcknowledged=true -> ACKNOWLEDGED
  • IsActive=false -> RESOLVED
  • otherwise -> FIRING
• Added provider docs: keep/providers/solarwinds_provider/README.md
• Added sample payload: keep/providers/solarwinds_provider/alerts_mock.py
• Added formatter tests: tests/test_solarwinds_provider.py

Quality notes

• Hardened parser for empty payloads (event = event or {}) to avoid runtime errors.
• Added fallback id/name behavior when key fields are missing.
• Local syntax check passed (python3 -m py_compile).