Add support for migration hooks

The current migration system limits what one can do in a schema migration to things that are expressible in EdgeQL. In order to enable more elaborate data migrations we should add support for attaching a Python or a JavaScript (or any other supported language) function to a migration and run it in the same transaction as the migration. Both pre- and post- migration hooks make sense, so we should support both.

The "same transaction" requirement means that the hook function would receive a transaction object as its argument. We would need the server to expose `pg_export_snapshot`, `SET TRANSACTION SNAPSHOT`, as well as two-phase commit, so `PREPARE TRANSACTION` and `COMMIT/ROLLBACK PREPARED`. These could be useful on their own in advanced scenarios.

Start by reading the migration system and its transaction handling; no specific files or tests are named in the issue. Clarify the design for attaching Python or JavaScript functions as pre- and post-migration hooks, then define how the transaction object, exported snapshots, and two-phase commit make same-transaction execution verifiable.

Written by the indexing model from the issue text.