Add AstraDB adapter

[Paristha]

Adds support for AstraDB.
Notes:

• Requires AstraDB database access token and database api endpoint
• Implements MaxSim--WARNING: No native support for MaxSim in AstraDB means a high number of api calls (one api call per pdf page per vector in the query's ColBERT embedding). This was not really an issue during testing on colab for pdfs of a few dozen pages, could be a problem at larger scales.