Add PostgresML as a datastore provider

[santiatpml]

PostgresML is added as a datastore provider. Using PostgresML, in addition to embeddings/vector search, users can enhance their text data by running NLP tasks like text classification, token classification etc. using HuggingFace transformers directly on the DB. Users can also run traditional ML algorithms on the DB to boost their queries. PostgresML also provides hosted databases with GPUs.

Here is a list of NLP tasks