Add ``executor`` configuration to each plugin.

[s0undt3ch]

Each plugin should be able to define the executor they should run on.

• cooperative: Using the async implementation, currently, asyncio.
• process: Deferring work to a process on a process pool.
• thread: deferring work to a thread in a thread pool.

Current problems to solve:

• The collectors return events using an async generator. How can we do the same when deferring to either a process or thread pool?
• The process/thread pools should be configurable, ie, how many processes/threads should the pool run.