Add equivalent to psycopg2 `register_adapter`

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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.

Start by reviewing asyncpg's existing set_type_codec support and psycopg2's register_adapter behavior. Determine how custom Python classes should be represented for query parameters without requiring PostgreSQL types, and define completion around equivalent add and select behavior without manual str() casts.

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