Mistral embedded mode

It'd be very convenient to be able to run Mistral components (engine and executor) as embedded into other applications so that we could call Mistral engine API directly from Python.

In order to do this we need to look at the following things:
* Will there be some limitations for host applications regarding using green threads (Mistral needs to run a number of things in green threads)
* Do we need to change the layout of mistral repo? Potentially we may need to split it into different repos.
* What will be the integration points with host applications?
* Clearly design how Mistral Engine API should look like in this case

This BP requires a detailed spec before starting with it.