Add commit-time actions for generating ABOUT files and attribution reports

[JonoYang]

Not sure if this is the best place to have this ticket, but it would be a good idea to have some mechanism that generates ABOUT files and attribution reports from the manifest files of a project. For example, I have a Python project and I commit some changes to my repo, I would like a commit-time action that would read the manifest files of my project, generate the ABOUT files for all of the dependencies found in my manifest files, then create an attribution report. The generated ABOUT files and attribution report would then be commited to the repo. We can use the scripts from scancode-toolkit/etc/release/ to perform the metadata retrieval/ABOUT file generation for Python projects, but we will need to extend it/create other tools for other languages.