fix: add missing frontmatter trigger args

[caterryan]

Summary

• Add missing HTTP frontmatter arg configs for downsampler, influxdb_to_iceberg, notifier, and prophet_forecasting.
• Fix SageMaker frontmatter wording so metadata parses from both raw source text and Python docstrings.
• Document Notifier nested sender configuration keys in the HTTP metadata.
• Update Downsampler README HTTP parameters for backfill_start/backfill_end and correct stale function references.