Support typed prepared statements

While datatypes are optional in prepared statements, not using them can results in type errors such as

function sum(text) does not exist

This is easily reproducible in any PSQL client

PREPARE my_query AS
SELECT SUM(CASE WHEN v = 'A' THEN $1 ELSE $2 END)
FROM (VALUES ('A'), ('B')) as q0(v);
-- ERROR: function sum(text) does not exist

But when I specify the types, it works

PREPARE my_query(int, int) AS
SELECT SUM(CASE WHEN v = 'A' THEN $1 ELSE $2 END)
FROM (VALUES ('A'), ('B')) as q0(v);

EXECUTE my_query(1,0);
-- 1

DEALLOCATE my_query;

asyncpg currently does not include types, which causes me to run into the sum(text) error. I'd expect it to either include types, or allow me to specify the types somehow.

Workaround:
Include a cast expression.
i.e.: $1::int or CAST($1 AS INT)

Full example

PREPARE my_query AS
SELECT SUM(CASE WHEN v = 'A' THEN $1::int ELSE $2::int END)
FROM (VALUES ('A'), ('B')) as q0(v);

まず、PostgreSQL の prepared statement の例を asyncpg 経由で再現し、パラメータ型がどのように扱われるかを調査します。示されているクエリを対象としたカバレッジを用意し、型付き prepared statement をサポートするか、呼び出し側がパラメータ型を指定できる方法を提供することで、報告されている SUM(text) の失敗に対処できれば完了です。