Add configurable value-remapping mixin

[msto]

Summary

The current Metric class has a _empty_field_to_none model validator that automatically converts empty string fields to None during parsing. This behavior is enabled by default for consistency with fgpyo's Metric, but could be improved with a more flexible, opt-in approach.

Context

#6 (comment)

It might be nice to have "" be a ClassVar so that users could override it with their own special "empty token".

The current implementation hardcodes the empty string to None conversion, which doesn't allow for:

1. Disabling the behavior when not needed
2. Customizing which values map to None
3. Supporting multiple input representations (e.g., both "" and "None" -> None)
4. Handling other sentinel values like "NA" -> np.nan

Suggested Solution

Create a mixin class (e.g., RemapValues) that:

1. Uses a dict class variable for arbitrary value mappings instead of a single "empty token"
2. Generalizes to support cases like:
   • multiple string representations of null: {"": None, "None": None}
   • mixed null/NaN handling: {"": None, "NA": np.nan}
3. (Maybe?) Is opt-in rather than enabled by default on the base Metric class