[META] Plugin improvements

[msm-code]

Plugins are currently a "secret knowledge", there's not a lot of documentation, and the existing documentation is outdated at some parts.

We should update the documentation, and open-source more useful components. I will keep updating this issue with more ideas as I get them. For now:

• Update plugin documentation to better reflect reality
• Open-source S3 plugin, and write a documentation on how to use it (and in general, how to use mquery with s3). Open-source and document the S3 plugin. #326