Multi - framework/platform support

This proposal allows us to move on from RxJava only implementation.

The general idea is to have a minimalistic "core" as a Kotlin MPP module with our own implementation of the base types: `Source`, `Consumer`, `Feature` and `Binder`.

The framework level contains a set of extensions to convert between "core" types to framework ones (e.g. from `Source` to `ObservableSource` from Rx and vice versa). In current [PoC](https://github.com/ShikaSD/multiK), we need to provide the following implementations:

- Conversion between `Source` and `Consumer` to framework types
- Feature implementation with framework source type (optional)
- Binder extensions for framework type (optional)

As we don't do any thread/concurrency management in the "core" implementation, I suspect we need to have some checks (similar to current `SameThreadVerifier`) in place to prevent race conditions.

Start by reading the linked multiK PoC and the existing RxJava-only implementation. Trace the Source, Consumer, Feature, Binder, and SameThreadVerifier types to understand the proposed core and framework boundaries. Done means the scope is agreed and the required conversions are supported without introducing concurrency races.

Written by the indexing model from the issue text.