feat: expose PgTypeInfo consts for implement Type outside

[tisonkun]

Currently, when you want to implement extend types:

use sqlx::{Postgres, Type};
use sqlx::postgres::PgTypeInfo;

pub struct Timestamp(jiff::Timestamp);

impl Type<Postgres> for Timestamp {
    fn type_info() -> PgTypeInfo {
        PgTypeInfo::TIMESTAMPTZ
    }
}

It will fail with Constant `TIMESTAMPTZ` is private [E0603].

These consts can be public for implement Type outside.

[tisonkun]

Closed -

There is: I don't want the docs page for PgTypeInfo to be a miles-long list of associated consts with its methods and trait impls buried at the very bottom.

Perhaps these could live in a postgres::types::catalog module, but I'd need to think on that. I'd want to do all of them at once, which is out-of-scope for this PR.

from #3511 (comment).