Configure index readers to read labels from config files

[mahesh-maan]

Currently, indexes are stored as two files: a binary index file (.ann or .faiss) and a json file. The json file contains a list of labels. It has been decided to include additional information in it along with the labels. Additional information could include, for example, the indexing parameters (metadata associated with the index).

The FaissIndexCreator has already been configured to create the new json file (now called config file, as opposed to previously, labels file). The FaissIndexReader and AnnoyIndexReader should also be updated to read this new format.