Custom Serialization (#486)

Add support for using custom serializers and deserializers in DBOS
Python. You can now supply a custom serializer in DBOS configuration,
which completely replaces the default `pickle`-based serializer for
storing objects in the system database.

For example, here's how you would use a JSON serializer:

```python
from dbos import DBOS, DBOSConfig, Serializer

class JsonSerializer(Serializer):
    def serialize(self, data: Any) -> str:
        return json.dumps(data)

    def deserialize(cls, serialized_data: str) -> Any:
        return json.loads(serialized_data)

serializer = JsonSerializer()
config: DBOSConfig = {
    "name": "dbos-starter",
    "system_database_url": os.environ.get("DBOS_SYSTEM_DATABASE_URL"),
    "serializer": serializer
}
DBOS(config=config)
DBOS.launch()
```

Addresses https://github.com/dbos-inc/dbos-transact-py/issues/485