Create registries for extension points

[PiotrSikora]

We need to provide registries in this repository for various extension points:

• settings (#55),
• properties (#60),
• custom hostcalls and callbacks, to assign enum-values and avoid duplications across implementations.

Also, SDKs should be able to query Host about them at startup, so that SDKs can refuse to start e.g. if a specific custom hostcall isn't available.