Define plugin requirements

We need to specify the licenses we permit. It seems reasonable to me to only permit open source licenses for plugins, but I don't think we've ever had that as a hard requirement.

Start by reviewing the issue and its three comments to understand the unresolved question about plugin licenses. The work is done when the permitted licenses are explicitly decided and the plugin requirements are specified; the issue does not mention files or tests to start from.

Written by the indexing model from the issue text.