Allow registering the dict->IContext adapters other than at import time.

Instead of just registering the adapters from `dict` to `IContext` and `ICheckpointable`, provide `AdaptationOffers` for both, and introduce a function to register them to an adaptation manager of choice.

This allows to use the adapters in a context where the adaptation manager is not the global one, or
where the adaptation manager is reset (e.g., unittesting).