Date strings are incorrectly returned as datetime objects after being stored and retrieved from Redis cache

[galvana]

Bug Description

A date string such as 2001-11-08 is converted to a datetime object after being stored and retrieved from the Redis cache.

Steps to Reproduce

This is easily reproduced by this test

def test_datetime_string_stays_string(self):
        date = {"birthday": "2001-11-08"}
        decoded = FidesopsRedis.decode_obj(FidesopsRedis.encode_obj(date))
        assert decoded == date

AssertionError: assert {'birthday': datetime.datetime(2001, 11, 8, 0, 0)} == {'birthday': '2001-11-08'}

Root cause

The _custom_decoder in cache.py converts any date string to a datetime object after it is read from the cache. This change was introduced as part of #2643

def _custom_decoder(json_dict: Dict[str, Any]) -> Dict[str, Any]:
    for k, v in json_dict.items():
        try:
            json_dict[k] = datetime.fromisoformat(v)
            continue
        except (TypeError, ValueError):
            pass

Expected behavior

The object should be stored and retrieved without converting the date string to a datetime object.

Potential fix

Store any date string with a prefix like we do when storing bytes or ObjectId so that they are not converted to datetime objects after being read from the cache.

Additional context

This issue has the potential to affect SaaS erasure requests since we can't build a valid payload to update a collection after the access request data has been stored and retrieved from the cache.
TypeError: Object of type datetime is not JSON serializable