Support collection.abc.Mapping as resource instead of only dict

[axelv]

This slight modification to the engine relaxes the assumption that a resource node must be a Python dictionary by changing
some isinstance checks:

- if isinstance(node.data, dict):
+ if isinstance(node.data, Mapping):

This allows us to pass Pydantic objects or dataclasses and preserve metadata and methods defined on those objects and solves the issue described here #43.

Let me know what you think and if adaptation are needed 🙌

FYI: I think a similar relaxation of list to Sequence would also be possible 🙂

[axelv]

@ruscoder What do you think of this approach?