Allow duckling to bind custom classes

[ltbringer]

Duckling's default class associations work fine but there cases where we need to create a custom class.

A case is to extend an entity to have a special method that computes time values differently (or say can add one time-entity with another). Here this would require a modification of existing types, breaking the extensibility.

[ltbringer]

This needs to undergo changes to make room for this request.

https://github.com/Vernacular-ai/dialogy/blob/c6c31519ae0f229c0d94ca086e6f5e6ac7091d3c/dialogy/types/entity/__init__.py#L16

[ltbringer]

A simple solution would be to expect users to create their own config that specifies modifications like:

dimension_entity_map = {
    "duration": {"value": CustomDurationEntity},
}

This can be passed to DucklingParser during instantiation, a dictionary merge can help drive the feature.