Fixes #1409

Changes Made

Created OpenSSF importer openssf.py and added files for testing the importer

Results

Ran the importer locally using command below with debug mode on
docker-compose exec vulnerablecode ./manage.py import vulnerabilities.importers.openssf.OpenSSFImporter

Importer ran successfully and imported 17182 advisories as can be seen in the image below

Other Consideration

1. Initially I named the file ossf.py but it was tantalizingly close to oss_fuzz.py and could cause confusion, hence changed the name to openssf.py. Let me know if any better name is possible
2. Out of the 4 ecosystems in the repository (crates.io, npm, pypi, rubygems), univers supports 3 (npm, pypi, rubygems), but I am not sure if it supports crates.io
3. For the time being, I have added the 4 ecosystems in the list, but if required I can create a separate classmethod called supported_ecosystems and add all the ecosystems there as a list/dictionary, similar to how it is done in Vulntotal. Do let me know what is best approach

@classmethod
    def supported_ecosystem(cls):
        return {           
            "pypi": "PIP",
            "gem": "RUBYGEMS",
            "npm": "NPM",           
        }

4. Have add a try-except block in for loop. Let me know if that has to be removed.