Add `SemanticSimilarity` metric into the `Langchain::Evals::Ragas` class namespace

[andreibondarev]

Spec: https://github.com/explodinggradients/ragas/blob/main/docs/concepts/metrics/available_metrics/semantic_similarity.md

[andreibondarev]

@bborn This is what I was thinking.

[bborn]

@andreibondarev something like this ... 3dee98f