Modify factories of opportunity attachment staging / foreign tables to be able to load files directly to the DB

[chouinar]

Summary

We want the factories for the opportunity attachment tables to be pre-populated with files stored in the columns when we generate them from our factories

An example program that does a bit of this manually with actual files (not a requirement):

def test_thing(db_session, enable_factory_create):
    # Reading a file and writing to the database
    # We should be able to do something like this in our factories
    # Potentially with raw text / byte streams instead
    with open("my_file", "rb") as outfile:
        opp = OpportunityFactory.create(my_attachment=outfile.read())
    opp_id = opp.opportunity_id
    # Force the next query to be from the DB for testing purposes
    db_session.commit()
    db_session.expire_all()
    db_opp = db_session.query(Opportunity).filter(Opportunity.opportunity_id == opp_id).one_or_none()
    # Attaching opportunity
    with open("out_file.txt", "wb") as outfile:
        outfile.write(db_opp.my_attachment)

Acceptance criteria

• Factories work to create dummy "files" in the staging and foreign DB tables
• The contents of the files don't really matter, feel free to generate the bytes on the fly. It's fine to turn a text string into bytes, so long as it can later be made into a file.