Add before_transaction/1 and after_transaction/1 migration callbacks

[a-nassim]

Closes https://groups.google.com/g/elixir-ecto/c/yLHOK68mAgo/m/ymbM0xFRAQAJ

Adds optional before_transaction/1 and after_transaction/1 migration callbacks that run around the migration transaction, on the same checked-out connection, for commands that only work outside one, like SQLite PRAGMA foreign_keys.

• after_transaction/1 always runs, also on rollback, so state can be restored (that's why it was not named after_commit). Like after_begin/0 and before_commit/0, both are skipped when there is no transaction.
• They receive the repo as an argument: they run in the migrator, where there is no runner, so repo/0, execute/1, flush/0 and direction/0 are unavailable.
• Alternative: moving the transaction into Ecto.Migration.Runner which would be more consistent with existing callbacks, but the change is a bit more involved, would you prefer that approach?

[josevalim]

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