docs: Document HTTP request parameters for downsampler and influxdb_to_iceberg plugins

## Summary

The `downsampler` and `influxdb_to_iceberg` plugins support both `scheduled` and `http` trigger types, but their HTTP request parameters are not formally documented in the plugin metadata or README files.

## Current State

- Both plugins have `scheduled_args_config` in their docstrings
- Neither has `http_args_config` for HTTP-specific parameters
- HTTP handlers accept parameters via request body that differ from scheduled trigger arguments

### downsampler

HTTP mode uses different/additional parameters:
- `backfill_start`, `backfill_end` instead of `window`
- `batch_size` for chunked processing
- `tag_values` as JSON object instead of string format

### influxdb_to_iceberg

Likely similar situation — needs code review to identify HTTP-specific parameters.

## Recommended Actions

1. Review `process_request` functions in both plugins to identify all HTTP parameters
2. Add `http_args_config` to plugin docstrings for Explorer UI support
3. Add `### HTTP request parameters` section to READMEs following the pattern in `prophet_forecasting`

## Related

- Prophet forecasting already has separate sections: `### Scheduled trigger parameters` and `### HTTP request parameters`
- See commit ab1b21f for the pattern to follow

Review the process_request functions and existing scheduled_args_config docstrings in the downsampler and influxdb_to_iceberg plugins to identify HTTP-specific parameters. Compare the documentation structure with prophet_forecasting and commit ab1b21f. Done means both plugins have http_args_config metadata and README sections documenting their HTTP request parameters.

Written by the indexing model from the issue text.