Add API server config.download functionality.

[hacktobeer]

Description of the change

This PR adds a config.download API end point to download the Turbinia configuration file in it's original format.

Applicable issues

• fixes #1514

Additional information

Checklist

• [x] All tests were successful.
• [x] Unit tests added.
• [ ] Documentation updated.

[hacktobeer]

@jleaniz first API server contribution. Can you have a look and see what needs to be added/changed? Probably:

• unit tests
• do I need to generate the client? if so, how?

[hacktobeer]

@jleaniz PTAL -> tests added and rewritten to use FileResponse fastapi function.