Introduce DynamicMapAttribute

[jpinner-lyft]

DynamicMapAttribute is a map attribute that supports declaring attributes (like an AttributeContainer),
but will also store any other values that are set on it (like a raw MapAttribute).

This class lets you create a "partially typed" map attribute so that it can store items that are not just primitive python types.

>>> from datetime import datetime
>>> from pynamodb.attributes import DynamicMapAttribute
>>> from pynamodb.attributes import UTCDateTimeAttribute
>>> from pynamodb.models import Model
>>> 
>>> class CreatedAtMap(DynamicMapAttribute):
...     created_at = UTCDateTimeAttribute()
... 
>>> class MyModel(Model):
...     my_map = CreatedAtMap(default={})
... 
>>> my_model = MyModel()
>>> my_model.my_map.created_at = datetime.utcnow()
>>> my_model.my_map.foo = 'bar'
>>> my_model.serialize()
{'my_map': {'M': {'created_at': {'S': '2020-10-14T16:39:15.077254+0000'}, 'foo': {'S': 'bar'}}}}
>>> 
>>> my_model_2 = MyModel()
>>> my_model_2.deserialize({'my_map': {'M': {'created_at': {'S': '2020-10-14T16:39:15.077254+0000'}, 'foo': {'S': 'bar'}}}})
>>> my_model_2.my_map.created_at
datetime.datetime(2020, 10, 14, 16, 39, 15, 77254, tzinfo=datetime.timezone.utc)
>>> my_model_2.my_map.foo
'bar'

[jmphilli]

this blob transforms a raw dict into a map attr object right? is this not shared with raw map attrs?

[jmphilli]

this serializes every defined attribute

[jmphilli]

this loop serializes the left over, undefined attrs

[garrettheel]

Should we dedup this with MapAttribute.serialize?

[garrettheel]

We might also want to add a mention here: https://pynamodb.readthedocs.io/en/latest/attributes.html#map-attributes

[jmphilli]

add discriminator + dynamicmapattr test

[garrettheel]

This applies only to MapAttribute subclasses, right? Might want to reorder the DynamicMapAttribute content underneath so as not to confuse

[garrettheel]

Should we also test with a subclass of CreatedAtMap to test one more level of inheritance?

[garrettheel]

Note that this is a new branch that should always return True for MapAttribute and sometimes return True for DynamicMapAttribute. Calling it out because there's some perf impact, though I think it should be marginal