feat: Enable adding files

[mattzh72]

Description

Enable adding files (refactored from documents). Specifically, this PR:

• Defines data models for File objects
• Supports storing files in SQL databases
• Adds an API route to retrieve all the files associated with a source
• Add pagination to API route

Also, adds some items to the .gitignore.

I ran alembic via the instructions:

poetry run alembic upgrade head
poetry run alembic revision --autogenerate -m "Add FileMetadata table and drop Documents table"

Testing

Added a test that loads the memgpt paper, retrieves the documents via the new API route, and confirms that:

1. the correct number of files were made
2. the metadata contains the right file name
3. the sources are set correctly on the files

[sarahwooders]

We may want to rename this concept to a file, and associate it with an entire PDF (or file more generally) that gets uploaded.

[mattzh72]

cc @4shub for correctness - I noticed it didn't mention dropping the Documents table or adding a new FileMetadata table - is this expected?

[4shub]

This is correct, alembic only does any change on database deletions or modifications. You did not remove any database you just removed a scheme