Add command to load existing embeddings into a collection

[justaddcoffee]

See https://github.com/monarch-initiative/curate-gpt/pull/35

So for example you can do this:

curategpt embeddings index /path/to/local/embeddings.parquet

or this:

curategpt embeddings index https://huggingface.co/datasets/biomedical-translator/monarch_kg_embeddings/resolve/main/deepwalk_embedding.parquet?download=true -f parquet