Introduce a generic Min/Max/Mean (mmm) mechanism at the plugin model level.
When a field in `fields_description` defines `'mmm': True`, the plugin
automatically generates and maintains the following derived fields:
<field>_min
<field>_max
<field>_mean
The computation is handled in the base plugin model to ensure the
feature is reusable across all plugins. Mean is calculated as a
running mean, and min/max are updated on each refresh cycle.
This implementation:
- Keeps the feature opt-in per field
- Avoids hardcoding logic in individual plugins
- Maintains full backward compatibility
- Preserves existing API v4 response structure
- Ensures no regression in existing behavior
Unit tests have been added to validate correct field generation
and update behavior.
