Add a way to add document_sources initially to ExplorerUI

[ahuang11]

Currently I have to insert in memory

import glob
import lumen.ai as lmai
from lumen.sources.duckdb import DuckDBSource
from lumen.ai.memory import memory


document_sources = []
documents = sorted(glob.glob("*_metadata.txt"))
for document in documents:
    with open(document, "r") as f:
        text = f.read()
    metadata = {"source": document}
    document = {"text": text, "metadata": metadata}
    document_sources.append(document)
memory["document_sources"] = document_sources