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.

首先检查 asyncpg 现有的 set_type_codec 支持以及 psycopg2 的 register_adapter 行为。确定自定义 Python 类应如何表示为查询参数，而不要求 PostgreSQL 类型，并以无需手动 str() 转换即可实现等效 add 和 select 行为作为完成标准。