Support typed prepared statements

Nobody has claimed this yet.

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

Start by reproducing the prepared-statement examples in PostgreSQL through asyncpg and inspect how parameter types are handled. Done means the reported SUM(text) failure is addressed by supporting typed prepared statements or a way for callers to specify parameter types, with coverage for the shown query.

Written by the indexing model from the issue text.