Add an MinvResult class to store minv results

# Feature

We need a result class for model inversion results.

## Desired Behavior / Functionality

This class should contain the relevant metrics. It should also be flexible enough to contain either image reconstruction metrics and reconstructed examples, or tabular reconstruction metrics or be adapted to other modalities.

## What Needs to Be Done

- Implement a class to handle these results
- Adapt run_attack in plgmi.py to return an instance of this class

## How Can It Be Tested

Run a plgmi attack with leakpro and make sure that it returns the correct thing