Add shared normalizer helpers to centralize payload extraction, JSON decoding,
and event finalization/validation.
Refactor NWS, Open-Meteo, and OpenWeather observation normalizers to use the
shared spine, removing repeated boilerplate while preserving provider-specific
mapping logic.
- Require params.user_agent for all HTTP sources (uniform config across providers)
- Add common.RequireHTTPSourceConfig() to validate name/url/user_agent in one call
- Add common.NewHTTPClient() with DefaultHTTPTimeout for consistent client setup
- Add common.SingleRawEvent() to centralize event envelope construction + validation
- Refactor NWS/Open-Meteo/OpenWeather observation sources to use new helpers
