add pdf loader

[zzstoatzz]

may also want a bulk pdf loader at some point to avoid extra chroma clients

closes #161

import asyncio

async def main():
    remote_pdf_document = await PDFLoader(
        file_path="https://www.cs.cmu.edu/~jgc/publication/The_Use_MMR_Diversity_Based_LTMIR_1998.pdf"
    ).load()

    local_pdf_document = await PDFLoader(
        file_path="/Users/nate/Downloads/MMR.pdf"
    ).load()
    assert len(remote_pdf_document) == len(local_pdf_document)

    assert remote_pdf_document[0].text == local_pdf_document[0].text
    assert remote_pdf_document[0].order == local_pdf_document[0].order

    assert remote_pdf_document[-1].text == local_pdf_document[-1].text
    assert remote_pdf_document[-1].order == local_pdf_document[-1].order

asyncio.run(main())

[jlowin]

This would be a good place to warn about needing to install marvin[pdf]