Support exporting index to HuggingFace Hub

Indexing is time consuming, and oftentimes people would like to be able to easily share pre-built index for various common datasets, for general domain application (wikipedia, code documentation...) and evaluation purposes.

A simple way to support this would be to add a `util` function that'd export the full index folder to the huggingface model, effectively exporting both the ColBERT config + the compressed vectors, allowing you to to do something like `RAGPretrainedModel.from_prebuilt_index("EXAMPLE_USER/Wikipedia")` and immediately begin querying the index.

Start by locating the utility functions that create or load a full index folder and the RAGPretrainedModel.from_prebuilt_index entry point. Check how ColBERT configuration and compressed vectors are stored; done means an index can be exported to HuggingFace Hub and loaded from a repository such as EXAMPLE_USER/Wikipedia for querying.

Written by the indexing model from the issue text.