RFE: Implement a plugin-level description metadata item

[dustinblack]

Please describe what you would like to see in this project

We currently have description metadata for steps and for schema objects. I would like to see us add a plugin-level description metadata item so that there is one authoritative place to query a plugin for its description. This will be valuable in the implementation of a registry and with arcalot/arcaflow-docsgen#46