stdlib: Add the GetTxFromDriverTx method

[CyJaySong]

In the compatibility mode of the database/sql standard library, obtain pgx.Tx

[jackc]

At first glance, the code seems fine. However, it should be tested. And is there some particular use case this enables?

[CyJaySong]

At first glance, the code seems fine. However, it should be tested. And is there some particular use case this enables?

use case for river with bun， bun uses pgx/stdlib to connect to the database, while river uses pgx. To ensure consistency, it is necessary to obtain pgx.Tx from the transaction of bun.Tx

I have already used it in a production environment. This code is simple enough, so I don't intend to write tests for it. It's unlikely to go wrong.

[CyJaySong]

@jackc please