Summarize API should be able to produce valid SPDX files

Input:
* Optional param in the summarize API request to request for SPDX format.

Output:
* Valid SPDX file with minimal datapoint (Source URL, License, Copyright plus minimal set of required fields to be a valid SPDX file.

Side-notes:
* Thomas is working on minimizing the set of required fields

The issue does not name files, tests, or an entry point. Start by locating the summarize API request and response implementation, then check the SPDX requirements referenced in the issue; done means the optional request produces a valid SPDX file containing the stated source URL, license, copyright, and required fields.

Written by the indexing model from the issue text.