Add a Downstream Validation Task

[Pale-Blue-Dot-97]

Is your feature request related to a problem? Please describe.
There is limited support currently within minerva for proper downstream validation of a model as it is being fitted. This is particularly problematic for self-supervised training as the current Weighted KNN validation is a poor test of the model's true performance.

Describe the solution you'd like
A custom task that extracts the encoder, turns its gradients off, builds a model around the encoder then trains this model on a downstream task for several epochs, then tests it with separate validation data and logs the result.

Describe alternatives you've considered
Continue to rely on the Weighted KNN.