Add support for AstraDB Vector Store

[willbakst]

Is your feature request related to a problem? Please describe.
Not a problem, just a feature request. Would be nice to support AstraDB for easier usage with Mirascope.

Describe the solution you'd like
Implementation of an AstraVectorStore like the already supported PineconeVectorStore and ChromaVectorStore.

Describe alternatives you've considered
N/A beyond using the AstraDB package directly.

[willbakst]

@Tylersuard please comment on this issue so that I can assign it to you (as you've already begun work on this in PR #197)

[willbakst]

@Tylersuard has mentioned separately that he unfortunately does not have enough time to complete this and would prefer we take it over the finish line on his behalf.

[willbakst]

We do not currently have any plans on bringing RAG out of beta (except possibly something with Postgres, but not sure), so I'm closing this issue until there's a strong reason to implement such an integration.