Big refactor to make PG modular

[anubhuti24]

• instructor through litellm
• embeddings through litellm
• use arbitrary LM and embedding model
• ollama support for instructor
• Support multiple vector stores
• Support multiple databases
• Use local db

closes #22

[sutyum]

In general, one recurring issue is that there are lots of objects being created for the embeddings api client and the llm api client, while one only needs one of each.

Both of these should essentially be created based upon whatever the user wants when creating the Graph object.