Add equivalent to psycopg2 `register_adapter`

In psycopg2, register_adapter along with utility functions provides a useful way to map custom Python classes into valid representations, often by relatively simple behaviors such as casting to str. There is no equivalent in asyncpg meaning moving from psycopg2 to asyncpg requires adding lots of str() casts to add and select queries.

The closest equivalent is set_type_codec but this only works for types that are registered with Postgres, not custom Python classes.

Comienza revisando la compatibilidad existente de asyncpg con set_type_codec y el comportamiento de psycopg2.register_adapter. Determina cómo deben representarse las clases de Python personalizadas para los parámetros de consulta sin requerir tipos de PostgreSQL, y define la finalización en torno a un comportamiento equivalente de add y select sin conversiones manuales con str().