Splitting this from #165

We need to cache what each plugin supports. Basically if it has support for 1) scheduled functions runEveryX 2) event processing functions processEvent / processEventBatch. This lets us separate VMs better in the future. For example having a separate pool of piscina workers for scheduled tasks, starting there only the VMs that are relevant. This caching could be run when a plugin is installed.

It would help with distributing and optimising worker pools inside the plugin server, if we knew what each plugin can do. Basically, what it exports. This knowledge can be used to help make a nicer interface (no need to reorder export plugins that run at the same time) and it'll help us split processEvent/onEvent tasks (which need to finish very fast) from the rest (scheduled jobs and other async tasks)