Create registries for extension points #71

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

• settings (Allow hosts to advertise features/settings at runtime #55),
• properties (Use enums where possible #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.