Support get_by_ids

[nickderobertis]

Closes #41

• Implements the get_by_ids method, using JSON.MGET for JSON storage and pipeline + HGETALL for hash storage.
• The doc on the method is copied straight from the langchain base class
• Adds integration tests for JSON and hash data types
• I tried to add a unit test, but seems like I would have to mock a lot of the Redis library to make it work right. I ultimately removed it in the last commit. Let me know if this is a blocker. Maybe we could consider some kind of intermediate layer in between the vector store and Redis to make mocking easier.

[bsbodden]

@nickderobertis Currently evaluating... We'll let you know soon. Thanks for the contribution

[bsbodden]

LGTM, we'll work on a proper unit test. Mocking is a PITA. Thanks for the contribution.