plugin architecture: define file/directory format

**[ @mmguero](https://github.com/mmguero)** cloned issue [idaholab/Malcolm#578](https://github.com/idaholab/Malcolm/issues/578) on 2024-10-01:

> This is a sub-issue of **Malcolm "plugin architecture"** #399
>
>
>
> For the plugin types where it's applicable, we need to define some sort of a file/directory format. Ideas might include:
>
>
>
> * a single directory where you can drop all of the plugins
>
> * bind-mounted into the relevant (or all?) docker containers
>
> * a compressed file format (zip, 7z, gz, xz, whatever) containing the contents of the plugin
>
> * a `config.yml` file or something like it defining the type of plugin and any other information it might need
>
> * some sort of shared script or something added to the entrypoint chain of the docker images to extract the `config.yml` file from the compressed plugin file and decide if it's something this container needs to do
>
>

Start with the parent plugin architecture issue #399 and review how Docker containers and images currently receive shared files or entrypoint logic. Compare the proposed directory, compressed archive, and config.yml approaches; done means the plugin format, plugin type metadata, and container loading behavior are explicitly defined.

Written by the indexing model from the issue text.