`filewriter`: monitors

This regards nexus groups like:
- `raw_data_1/monitor_`
- `raw_data_1/monitor_events_not_saved`
- monitor events

At the streaming level, monitor data will be in event mode. For monitor events, we can then choose how to expose this data in the filewriter, as events or histograms or both. We will likely end up wanting all options (configurable).

It is possible that support for streamed-histograms (e.g. a histogram every N frames, where N might be 1 in the limit) will be added to the hardware later, but this is not currently implemented.

## Potential differences from existing files

No format changes expected where event/histogram configuration is equivalent to before.

Start by locating the filewriter implementation and its existing handling for the listed Nexus groups: raw_data_1/monitor_, raw_data_1/monitor_events_not_saved, and monitor events. Review how equivalent event and histogram configurations are currently exposed, then clarify the configurable output options and their expected behavior before implementation; streamed histograms are not currently available in hardware.

Written by the indexing model from the issue text.