These structs are intended for internal use only for the backend, and are
not intended to be used externally.
This moves the plugin-related `PluginRmConfig`, `PluginEnableConfig`, and
`PluginDisableConfig` types to the backend package to prevent them being
imported in the client, and to make it more clear that this is part of
internal APIs, and not public-facing.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>